Problem p02691 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <array>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctype.h>
#include <deque>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <string.h>
#include <unordered_map>
#include <utility>
#include <vector>

#define _USE_MATH_DEFINES
#include <iostream>
#include <math.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ll, double> pld;
typedef pair<double, double> pdd;
typedef pair<double, ll> pdl;
typedef pair<int, char> pic;
typedef vector<ll> vl;
typedef vector<int> vi;
typedef priority_queue<ll, vector<ll>, greater<ll>> llgreaterq;
typedef priority_queue<pll, vector<pll>, greater<pll>> pllgreaterq;
typedef priority_queue<pair<ll, pll>, vector<pair<ll, pll>>,
                       greater<pair<ll, pll>>>
    plpllgreaterq;
typedef priority_queue<vi, vector<vi>, greater<vi>> vigreaterq;
typedef priority_queue<vl, vector<vl>, greater<vl>> vlgreaterq;
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, -1, 0, 1};
#define bit(x, v) ((ll)x << v)
#define rep(x, n) for (ll x = 0; x < n; x++)
#define rep2(x, f, v) for (ll x = f; x < v; x++)
#define repe(v, x) for (auto v : x)
// 許容する誤差ε
#define EPS (1e-10)
// 2つのスカラーが等しいかどうか
#define EQ(a, b) (std::abs(a - b) < EPS)
// 2つのベクトルが等しいかどうか
#define EQV(a, b) (EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()))
#define all(a) a.begin(), a.end()
#define all0(a) memset(a, 0, sizeof(a))
#define allm1(a) memset(a, -1, sizeof(a))
#define put_float(v)                                                           \\
  cout << fixed << setprecision(10);                                           \\
  cout << v << endl
#define vinsert(v, p, x) v.insert(v.begin() + p, x)
#define vsort(v) sort(all(v));
#define dup(v) v.erase(unique(all(v)), v.end())
#define ion(i, j) ((i & (1LL << j)) > 0)
#define next(i)                                                                \\
  i++;                                                                         \\
  i %= 2
#define Len size()
#define ull unsignd long long
#define psp(a, b) push_back(make_pair(a, b))
#define psp2(a, b) push(make_pair(a, b))

const ll INF = 1000000007;
const int MAX = 2000010;
const int MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

ll getpow(ll b, ll x, ll md) {
  ll t = b;
  ll res = 1;
  while (x > 0) {
    if (x & 1) {
      res *= t;
      res %= md;
    }
    x >>= 1;
    t *= t;
    t %= md;
  }
  return res;
}
ll getpow(ll b, ll x) { return getpow(b, x, INF); }
ll modinv(ll x) { return getpow(x, INF - 2); }
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int pr[100010];
int lank[100010];
void uini(int n) {
  for (size_t i = 0; i <= n; i++) {
    pr[i] = i;
    lank[i] = 1;
  }
}

int parent(int x) {
  if (x == pr[x])
    return x;
  return pr[x] = parent(pr[x]);
}

int same(int x, int y) { return parent(x) == parent(y); }

bool unit(int x, int y) {

  int px = parent(x);
  int py = parent(y);

  if (px == py)
    return false;
  if (lank[py] < lank[px]) {
    pr[py] = px;
    lank[px] += lank[py];
  } else {
    pr[px] = py;
    lank[py] += lank[px];
  }
  return true;
}

ll merge(ll *a, int left, int mid, int right) {
  ll n1 = mid - left;
  ll n2 = right - mid;
  vector<int> L(n1 + 1);
  vector<int> R(n2 + 1);
  for (size_t i = 0; i < n1; i++) {
    L[i] = a[left + i];
  }
  for (size_t i = 0; i < n2; i++) {
    R[i] = a[mid + i];
  }

  L[n1] = INF;
  R[n2] = INF;
  ll i = 0;
  ll j = 0;
  ll r = 0;
  for (size_t k = left; k < right; k++) {
    if (L[i] <= R[j]) {
      a[k] = L[i];
      i++;
    } else {
      a[k] = R[j];
      r += n1 - i;
      j++;
    }
  }
  return r;
}
ll merge2(pair<int, char> *a, int left, int mid, int right) {
  ll n1 = mid - left;
  ll n2 = right - mid;
  vector<pair<int, char>> L(n1 + 1);
  vector<pair<int, char>> R(n2 + 1);
  for (size_t i = 0; i < n1; i++) {
    L[i] = a[left + i];
  }
  for (size_t i = 0; i < n2; i++) {
    R[i] = a[mid + i];
  }

  L[n1] = make_pair(INF, ' ');
  R[n2] = make_pair(INF, ' ');
  ll i = 0;
  ll j = 0;
  ll r = 0;
  for (size_t k = left; k < right; k++) {
    if (L[i].first <= R[j].first) {
      a[k] = L[i];
      i++;
    } else {
      a[k] = R[j];
      r += n1 - i;
      j++;
    }
  }
  return r;
}
ll mergeSort2(pair<int, char> *a, int left, int right) {
  ll res = 0;
  if (left + 1 < right) {
    int mid = (left + right) / 2;
    res = mergeSort2(a, left, mid);
    res += mergeSort2(a, mid, right);
    res += merge2(a, left, mid, right);
  }
  return res;
}
ll mergeSort(ll *a, int left, int right) {
  ll res = 0;
  if (left + 1 < right) {
    int mid = (left + right) / 2;
    res = mergeSort(a, left, mid);
    res += mergeSort(a, mid, right);
    res += merge(a, left, mid, right);
  }
  return res;
}
int partition(pair<int, char> *a, int p, int r) {
  pair<int, char> x = a[r];
  int i = p - 1;
  for (size_t j = p; j < r; j++) {
    if (a[j].first <= x.first) {
      i++;
      swap(a[i], a[j]);
    }
  }
  swap(a[i + 1], a[r]);
  return i + 1;
}
void quick(pair<int, char> *a, int p, int r) {
  if (p < r) {
    int q = partition(a, p, r);
    quick(a, p, q - 1);
    quick(a, q + 1, r);
  }
}

ll n;
int ci = 0;
ll P[1000010];
struct Node {
  int key;
  int priority;
  Node *parent, *left, *right;
  Node(int key, int priority);
  Node() {}
};
Node NIL;
Node::Node(int key, int priority) : key(key), priority(priority) {
  left = &NIL;
  right = &NIL;
}
Node *root = new Node();
void cenrec(Node *k) {
  if (k->key == NIL.key)
    return;
  cenrec(k->left);
  cout << " " << k->key;
  cenrec(k->right);
}
void fastrec(Node *k) {
  if (k->key == NIL.key)
    return;
  cout << " " << k->key;
  fastrec(k->left);
  fastrec(k->right);
}
void insert(Node *v) {
  Node *y = &NIL;
  Node *x = root;
  while (x->key != NIL.key) {
    y = x;
    if (v->key < x->key) {
      x = x->left;
    } else {
      x = x->right;
    }
  }
  v->parent = y;
  if (y->key == NIL.key) {
    root = v;
  } else if (v->key < y->key) {
    y->left = v;
  } else {
    y->right = v;
  }
}

Node *find(Node *k, ll v) {
  if (k->key == NIL.key)
    return &NIL;
  if (k->key == v)
    return k;
  if (v < k->key)
    return find(k->left, v);
  return find(k->right, v);
}
void delp12(Node *x) {
  if (x->key == NIL.key)
    return;
  Node *l = x->left;
  Node *r = x->right;
  Node *pr = x->parent;

  if (l->key == NIL.key && r->key == NIL.key) {
    if (pr->left == x) {
      pr->left = &NIL;
    } else
      pr->right = &NIL;
  } else if (l->key != NIL.key) {
    if (pr->left == x) {
      pr->left = l;
    } else
      pr->right = l;
    l->parent = pr;
  } else if (r->key != NIL.key) {
    if (pr->left == x) {
      pr->left = r;
    } else
      pr->right = r;
    r->parent = pr;
  }
}
Node *get_next(Node *k) {
  if (k->key == NIL.key)
    return &NIL;
  Node *res = get_next(k->left);
  if (res->key != NIL.key)
    return res;
  return k;
}
void del(Node *x) {

  if (x->key == NIL.key)
    return;
  Node *l = x->left;
  Node *r = x->right;
  Node *pr = x->parent;

  if (l->key != NIL.key && r->key != NIL.key) {
    Node *nex = get_next(r);
    x->key = nex->key;
    delp12(nex);
  } else {
    delp12(x);
  }
}
Node *rightRotate(Node *t) {
  Node *s = t->left;
  t->left = s->right;
  s->right = t;
  return s;
}
Node *leftRotate(Node *t) {
  Node *s = t->right;
  t->right = s->left;
  s->left = t;
  return s;
}
Node *_insert(Node *t, int key, int priority) {
  if (t->key == NIL.key) {
    return new Node(key, priority);
  }
  if (key == t->key) {
    return t;
  }

  if (key < t->key) {
    t->left = _insert(t->left, key, priority);
    if (t->priority < t->left->priority) {
      t = rightRotate(t);
    }
  } else {
    t->right = _insert(t->right, key, priority);
    if (t->priority < t->right->priority) {
      t = leftRotate(t);
    }
  }
  return t;
}
Node *delete1(Node *t, int key);
Node *_delete(Node *t, int key) {
  if (t->left->key == NIL.key && t->right->key == NIL.key) {
    return &NIL;
  } else if (t->left->key == NIL.key) {
    t = leftRotate(t);
  } else if (t->right->key == NIL.key) {
    t = rightRotate(t);
  } else {
    if (t->left->priority > t->right->priority) {
      t = rightRotate(t);
    } else
      t = leftRotate(t);
  }
  return delete1(t, key);
}
Node *delete1(Node *t, int key) {
  if (t->key == NIL.key) {
    return &NIL;
  }
  if (key < t->key) {
    t->left = delete1(t->left, key);
  } else if (key > t->key) {
    t->right = delete1(t->right, key);
  } else
    return _delete(t, key);
  return t;
}
int H;
int left(int i) { return i * 2 + 1; }
int right(int i) { return i * 2 + 2; }
struct edge {
  int from, to;
  ll val;
  edge(int from, int to, ll val) : from(from), to(to), val(val) {}
};
ll k;
int _rank[1010];
int temp[1010];
bool compare_sa(int i, int j) {
  if (_rank[i] != _rank[j])
    return _rank[i] < _rank[j];
  else {
    int ri = i + k <= n ? _rank[i + k] : -1;
    int rj = j + k <= n ? _rank[j + k] : -1;
    return ri < rj;
  }
}
void construct_sa(string S, int *sa) {
  n = S.length();

  for (size_t i = 0; i <= n; i++) {
    sa[i] = i;
    _rank[i] = i < n ? S[i] : -1;
  }

  for (k = 1; k <= n; k *= 2) {
    sort(sa, sa + n + 1, compare_sa);

    // saはソート後の接尾辞の並びになっている、rankは元の位置のindexを保持したまま、更新されている。
    // ピンとこなかった部分
    temp[sa[0]] = 0;
    for (size_t i = 1; i <= n; i++) {
      temp[sa[i]] = temp[sa[i - 1]] + (compare_sa(sa[i - 1], sa[i]) ? 1 : 0);
    }
    for (size_t i = 0; i <= n; i++) {
      _rank[i] = temp[i];
    }
  }
}
bool contain(string S, int *sa, string T) {
  int a = 0, b = S.length();
  // sa は 接尾辞が辞書順に並んでいる、入っているのはその位置のインデックス
  while (b - a > 1) {
    int c = (a + b) / 2;
    if (S.compare(sa[c], T.length(), T) < 0)
      a = c;
    else
      b = c;
  }
  return S.compare(sa[b], T.length(), T) == 0;
}

#define bit(x, v) ((ll)x << v)

class BIT {

  static const int MAX_N = 500010;

public:
  BIT() { memset(bit, 0, sizeof(bit)); }
  ll bit[MAX_N + 1], n;
  ll sum(int i) {
    ll s = 0;
    while (i > 0) {
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }

  void add(int i, int x) {
    while (i <= n) {
      bit[i] += x;
      i += i & -i;
    }
  }
};
struct UnionFind {
  vector<int> A;
  UnionFind(int n) : A(n, -1) {}
  int find(int x) {
    if (A[x] < 0)
      return x;
    return A[x] = find(A[x]);
  }
  void unite(int x, int y) {
    x = find(x), y = find(y);
    if (x == y)
      return;
    if (A[x] > A[y])
      swap(x, y);
    A[x] += A[y];
    A[y] = x;
  }
  int ngroups() {
    int ans = 0;
    for (auto a : A)
      if (a < 0)
        ans++;
    return ans;
  }
};
void yes() {
  cout << "Yes\\n";
  exit(0);
}
void no() {
  cout << "No\\n";
  exit(0);
}
vector<ll> getp(ll n) {

  vector<ll> res;
  ll a = 2;
  if (n % 2 == 0) {
    res.push_back(2);
    while (n % 2 == 0)
      n /= 2;
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {
      res.push_back(i);
      while (n % i == 0)
        n /= i;
    }
  }
  if (n != 1)
    res.push_back(n);
  return res;
}
vector<ll> getp2(ll n) {

  vector<ll> res;
  ll a = 2;
  if (n % 2 == 0) {

    while (n % 2 == 0) {
      n /= 2;
      res.push_back(2);
    }
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {

      while (n % i == 0) {
        n /= i;
        res.push_back(i);
      }
    }
  }
  if (n != 1)
    res.push_back(n);
  return res;
}
vector<pll> getp3(ll n) {
  vector<pll> res;
  ll a = 2;
  int si = 0;
  if (n % 2 == 0) {

    res.push_back(make_pair(2, 0));
    while (n % 2 == 0) {
      n /= 2;
      res[si].second++;
    }
    si++;
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {
      res.push_back(make_pair(i, 0));
      while (n % i == 0) {
        n /= i;
        res[si].second++;
      }
      si++;
    }
  }
  if (n != 1) {
    res.push_back(make_pair(n, 1));
  }
  return res;
}

vector<ll> getDivisors(ll n) {

  vector<ll> res;
  ll a = 2;
  res.push_back(1);
  for (ll i = 2; i * i <= n; i++) {

    if (n % i == 0) {
      res.push_back(i);
      if (n / i != i)
        res.push_back(n / i);
    }
  }
  return res;
}

struct ve {
public:
  vector<ve> child;
  int _t = INF;
  ve(int t) : _t(t) {}
  ve(ve _left, ve _right) {
    _t = _left._t + _right._t;
    child.push_back(_left);
    child.push_back(_right);
  }
  bool operator<(const ve &t) const { return _t > t._t; }
};

vector<bool> elas(ll n) {
  vector<bool> r(n);
  fill(r.begin(), r.end(), 1);
  r[0] = 0;
  r[1] = 0;
  for (ll i = 2; i * i < n; i++) {
    if (!r[i])
      continue;
    ll ti = i * 2;
    while (ti < n) {
      r[ti] = false;
      ti += i;
    }
  }
  return r;
}
bool isPrime(ll v) {
  for (ll i = 2; i * i <= v; i++) {
    if (v % i == 0)
      return false;
  }
  return true;
}

class SegTree {

public:
  const static int MAX_N = 100010;
  const static int DAT_SIZE = (1 << 18) - 1;
  int N, Q;
  int A[MAX_N];

  ll data[DAT_SIZE], datb[DAT_SIZE];
  void init(int _n) {
    N = 1;
    while (N < _n)
      N <<= 1;
    memset(data, 0, sizeof(data));
    memset(datb, 0, sizeof(datb));
  }
  void init(int _n, ll iv) {
    N = 1;
    while (N < _n)
      N <<= 1;
    rep(i, DAT_SIZE) {
      data[i] = iv;
      datb[i] = iv;
    }
  }
  void add(int a, int b, int x) { add(a, b + 1, x, 0, 0, N); }
  void add(int a, int b, int x, int k, int l, int r) {
    if (a <= l && r <= b) {
      data[k] += x;
    } else if (l < b && a < r) {
      datb[k] += (min(b, r) - max(a, l)) * x;
      add(a, b, x, k * 2 + 1, l, (l + r) / 2);
      add(a, b, x, k * 2 + 2, (l + r) / 2, r);
    }
  }

  void change(int a, int b, int x) { change(a, b + 1, x, 0, 0, N); }
  void change(int a, int b, int x, int k, int l, int r) {
    if (a <= l && r <= b) {
      data[k] = x;
    } else if (l < b && a < r) {
      datb[k] = x;
      change(a, b, x, k * 2 + 1, l, (l + r) / 2);
      change(a, b, x, k * 2 + 2, (l + r) / 2, r);
    }
  }

  ll sum(int a, int b) { return sum(a, b + 1, 0, 0, N); }
  ll sum(int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return 0;
    }
    if (a <= l && r <= b) {
      return data[k] * (r - l) + datb[k];
    }

    ll res = (min(b, r) - max(a, l)) * data[k];
    res += sum(a, b, k * 2 + 1, l, (l + r) / 2);
    res += sum(a, b, k * 2 + 2, (l + r) / 2, r);
    return res;
  }
};
class Segment;
class Circle;

class Point {
public:
  double x, y;

  Point(double x = 0, double y = 0) : x(x), y(y) {}

  Point operator+(Point p) { return Point(x + p.x, y + p.y); }
  Point operator-(Point p) { return Point(x - p.x, y - p.y); }
  Point operator*(double a) { return Point(a * x, a * y); }
  Point operator/(double a) { return Point(x / a, y / a); }

  double abs() { return sqrt(norm()); }
  double norm() { return x * x + y * y; }

  bool operator<(const Point &p) const { return x != p.x ? x < p.x : y < p.y; }
  bool operator==(const Point &p) const {
    return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
  }
  static double dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }
  static double cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }
  static bool isOrthogonal(Point a, Point b) { return EQ(dot(a, b), 0.0); }
  static bool isOrthogonal(Point a1, Point a2, Point b1, Point b2) {
    return isOrthogonal(a1 - a2, b1 - b2);
  }
  static bool isOrthogonal(Segment s1, Segment s2);

  static bool isPalallel(Point a, Point b) { return EQ(cross(a, b), 0.0); }
  static bool isPalallel(Point a1, Point a2, Point b1, Point b2) {
    return isPalallel(a1 - a2, b1 - b2);
  }
  static bool isPalallel(Segment s1, Segment s2);

  static const int COUNTER_CLOCKWISE = 1;
  static const int CLOCKWISE = -1;
  static const int ONLINE_BACK = 2;
  static const int ONLINE_FRONT = -2;
  static const int ON_SEGMENT = 0;
  static int ccw(Point p0, Point p1, Point p2) {
    Point a = p1 - p0;
    Point b = p2 - p0;
    if (cross(a, b) > EPS)
      return COUNTER_CLOCKWISE;
    if (cross(a, b) < -EPS)
      return CLOCKWISE;
    if (dot(a, b) < -EPS)
      return ONLINE_BACK;
    if (a.norm() < b.norm())
      return ONLINE_FRONT;
    return ON_SEGMENT;
  }

  static bool intersect(Point p1, Point p2, Point p3, Point p4) {
    return (ccw(p1, p2, p3) * ccw(p1, p2, p4) <= 0 &&
            ccw(p3, p4, p1) * ccw(p3, p4, p2) <= 0);
  }
  static bool intersect(Segment s1, Segment s2);
  static Point project(Segment s, Point p);

  static Point reflect(Segment s, Point p);

  static Point getDistance(Point a, Point b) { return (a - b).abs(); }

  static double getDistanceLP(Segment s, Point p);

  static double getDistanceSP(Segment s, Point p);

  static double getDistance(Segment s1, Segment s2);

  static Point getIntersection(Segment s1, Segment s2);

  static pair<Point, Point> crossPoints(Circle c, Segment s);

  static int contains(vector<Point> g, Point p) {
    int n = g.size();
    bool x = false;
    rep(i, n) {
      Point a = g[i] - p, b = g[(i + 1) % n] - p;
      // 線の上に載っているか
      if (std::abs(cross(a, b)) < EPS && dot(a, b) < EPS)
        return 1;

      // pを基準として上下にあるか
      // または外積が正か?(→にあるか)
      if (a.y > b.y)
        swap(a, b);
      if (a.y < EPS && EPS < b.y && cross(a, b) > EPS)
        x = !x;
    }
    return x ? 2 : 0;
  }

  static vector<Point> andrewScan(vector<Point> s) {
    vector<Point> u, l;
    if (s.size() < 3)
      return s;
    sort(all(s));
    u.push_back(s[0]);
    u.push_back(s[1]);
    l.push_back(s[s.size() - 1]);
    l.push_back(s[s.size() - 2]);

    for (int i = 2; i < s.size(); i++) {

      for (int _n = u.size();
           _n >= 2 && ccw(u[_n - 2], u[_n - 1], s[i]) > CLOCKWISE; _n--) {
        u.pop_back();
      }
      u.push_back(s[i]);
    }

    for (int i = s.size() - 3; i >= 0; i--) {

      for (int _n = l.size();
           _n >= 2 && ccw(l[_n - 2], l[_n - 1], s[i]) > CLOCKWISE; _n--) {
        l.pop_back();
      }
      l.push_back(s[i]);
    }

    reverse(all(l));
    for (int i = u.size() - 2; i >= 1; i--) {
      l.push_back(u[i]);
    }

    return l;
  }
  void get_cin() { cin >> x >> y; }
};

class Segment {
public:
  Point p1, p2;
  Segment() {}
  Segment(Point p1, Point p2) : p1(p1), p2(p2) {}
  void get_cin() { cin >> p1.x >> p1.y >> p2.x >> p2.y; }
  Point p1tp2() { return p2 - p1; }
  Point p2tp1() { return p1 - p2; }
  double abs() { return std::abs(norm()); }
  double norm() { return (p2 - p1).norm(); }
};

bool Point::isOrthogonal(Segment s1, Segment s2) {
  return EQ(dot(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
bool Point::isPalallel(Segment s1, Segment s2) {
  return EQ(cross(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
bool Point::intersect(Segment s1, Segment s2) {
  return intersect(s1.p1, s1.p2, s2.p1, s2.p2);
}
Point Point::project(Segment s, Point p) {
  Point base = s.p2 - s.p1;
  double r = Point::dot(p - s.p1, base) / base.norm();
  return s.p1 + base * r;
}
Point Point::reflect(Segment s, Point p) { return (project(s, p) * 2) - p; }
double Point::getDistanceLP(Segment s, Point p) {
  return std::abs(cross(s.p2 - s.p1, p - s.p1) / (s.p2 - s.p1).abs());
}
double Point::getDistanceSP(Segment s, Point p) {
  if (dot(s.p2 - s.p1, p - s.p1) < 0.0)
    return (p - s.p1).abs();
  if (dot(s.p1 - s.p2, p - s.p2) < 0.0)
    return (p - s.p2).abs();
  return getDistanceLP(s, p);
}
double Point::getDistance(Segment s1, Segment s2) {
  if (intersect(s1, s2))
    return 0.0;
  return min({getDistanceSP(s1, s2.p1), getDistanceSP(s1, s2.p2),
              getDistanceSP(s2, s1.p1), getDistanceSP(s2, s1.p2)});
}

Point Point::getIntersection(Segment s1, Segment s2) {
  // (s1.p1 - s2.p1).norm()
  auto bs = s1.p2 - s1.p1;
  auto n1 = s2.p1 - s1.p1;
  auto n2 = s2.p2 - s1.p1;
  auto c1 = std::abs(cross(n1, bs)) / bs.norm();
  auto c2 = std::abs(cross(n2, bs)) / bs.norm();
  return s2.p1 + (s2.p2 - s2.p1) * (c1 / (c1 + c2));
  // c1:c2=t:1-t
  // c2t=(1-t)c1
  // t/(1-t)=c1/(c1+c2)
  //
}

double arg(Point p) { return atan2(p.y, p.x); }
Point polar(double a, double r) { return Point(cos(r) * a, sin(r) * a); }
class Circle {
public:
  Point c;
  double r;
  Circle(Point c = Point(), double r = 0.0) : c(c), r(r) {}
  void get_cin() { cin >> c.x >> c.y >> r; }
  static pair<Point, Point> getCrossPoints(Circle c1, Circle c2) {
    double d = (c1.c - c2.c).abs(); // 中心点どうしの距離
    double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d));
    double t = arg(c2.c - c1.c);
    return make_pair(c1.c + polar(c1.r, t + a), c1.c + polar(c1.r, t - a));
  }
};

pair<Point, Point> Point::crossPoints(Circle c, Segment s) {
  auto pp = project(s, c.c);
  auto f = (pp - c.c).norm();
  auto mu = sqrt(c.r * c.r - f);
  auto e = s.p1tp2() / s.p1tp2().abs();
  return make_pair(pp + e * mu, pp - e * mu);
}

ll divRm(string s, ll x) {

  ll r = 0;
  for (ll i = 0; i < s.size(); i++) {
    r *= 10;
    r += s[i] - '0';
    r %= x;
  }
  return r;
}
ll cmbi(ll x, ll b) {

  ll res = 1;
  for (size_t i = 0; i < b; i++) {
    res *= x - i;
    res %= INF;
    res *= inv[b - i];
    res %= INF;
  }
  return res;
}

double digsum(ll x) {
  ll res = 0;
  while (x > 0) {
    res += x % 10;
    x /= 10;
  }
  return res;
}
bool check_parindrome(string s) {
  int n = s.size();
  rep(i, n / 2) {
    if (s[i] != s[n - i - 1]) {
      return false;
    }
  }
  return true;
}

void solv() {
  cin >> n;
  ll a[200010];

  rep(i, n) cin >> a[i];
  ll aia[1000010];
  all0(aia);
  ll hf = 500000;
  rep(i, n) {
    if (a[i] > n - 1)
      continue;
    aia[a[i] + i + hf]++;
  }
  ll res = 0;
  rep(i, n) {
    if (a[i] > n - 1)
      continue;
    res += aia[i - a[i] + hf];
    if (a[i] + hf + i == i + a[i] - hf)
      res--;
  }
  cout << res << endl;
  // ll x;
  // cin >> x;
  // map<ll, ll> mp;
  // rep2(i, -100000, 10001) {
  //	mp[pow(i, 5)] = i;
  // }
  // for(ll b = -100000; b <= 100000; b++) {
  //	ll t = x + pow(b,5);
  //	if (mp.count(t) > 0) {
  //		cout << mp[t] << " " << b << endl;
  //		return;
  //	}
  // }
}
int main() {
  // COMinit();
  solv();
  return 0;
}

Error: 0

Fixed submission:

#include <algorithm>
#include <array>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctype.h>
#include <deque>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <string.h>
#include <unordered_map>
#include <utility>
#include <vector>

#define _USE_MATH_DEFINES
#include <iostream>
#include <math.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ll, double> pld;
typedef pair<double, double> pdd;
typedef pair<double, ll> pdl;
typedef pair<int, char> pic;
typedef vector<ll> vl;
typedef vector<int> vi;
typedef priority_queue<ll, vector<ll>, greater<ll>> llgreaterq;
typedef priority_queue<pll, vector<pll>, greater<pll>> pllgreaterq;
typedef priority_queue<pair<ll, pll>, vector<pair<ll, pll>>,
                       greater<pair<ll, pll>>>
    plpllgreaterq;
typedef priority_queue<vi, vector<vi>, greater<vi>> vigreaterq;
typedef priority_queue<vl, vector<vl>, greater<vl>> vlgreaterq;
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, -1, 0, 1};
#define bit(x, v) ((ll)x << v)
#define rep(x, n) for (ll x = 0; x < n; x++)
#define rep2(x, f, v) for (ll x = f; x < v; x++)
#define repe(v, x) for (auto v : x)
// 許容する誤差ε
#define EPS (1e-10)
// 2つのスカラーが等しいかどうか
#define EQ(a, b) (std::abs(a - b) < EPS)
// 2つのベクトルが等しいかどうか
#define EQV(a, b) (EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()))
#define all(a) a.begin(), a.end()
#define all0(a) memset(a, 0, sizeof(a))
#define allm1(a) memset(a, -1, sizeof(a))
#define put_float(v)                                                           \\
  cout << fixed << setprecision(10);                                           \\
  cout << v << endl
#define vinsert(v, p, x) v.insert(v.begin() + p, x)
#define vsort(v) sort(all(v));
#define dup(v) v.erase(unique(all(v)), v.end())
#define ion(i, j) ((i & (1LL << j)) > 0)
#define next(i)                                                                \\
  i++;                                                                         \\
  i %= 2
#define Len size()
#define ull unsignd long long
#define psp(a, b) push_back(make_pair(a, b))
#define psp2(a, b) push(make_pair(a, b))

const ll INF = 1000000007;
const int MAX = 2000010;
const int MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

ll getpow(ll b, ll x, ll md) {
  ll t = b;
  ll res = 1;
  while (x > 0) {
    if (x & 1) {
      res *= t;
      res %= md;
    }
    x >>= 1;
    t *= t;
    t %= md;
  }
  return res;
}
ll getpow(ll b, ll x) { return getpow(b, x, INF); }
ll modinv(ll x) { return getpow(x, INF - 2); }
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
int pr[100010];
int lank[100010];
void uini(int n) {
  for (size_t i = 0; i <= n; i++) {
    pr[i] = i;
    lank[i] = 1;
  }
}

int parent(int x) {
  if (x == pr[x])
    return x;
  return pr[x] = parent(pr[x]);
}

int same(int x, int y) { return parent(x) == parent(y); }

bool unit(int x, int y) {

  int px = parent(x);
  int py = parent(y);

  if (px == py)
    return false;
  if (lank[py] < lank[px]) {
    pr[py] = px;
    lank[px] += lank[py];
  } else {
    pr[px] = py;
    lank[py] += lank[px];
  }
  return true;
}

ll merge(ll *a, int left, int mid, int right) {
  ll n1 = mid - left;
  ll n2 = right - mid;
  vector<int> L(n1 + 1);
  vector<int> R(n2 + 1);
  for (size_t i = 0; i < n1; i++) {
    L[i] = a[left + i];
  }
  for (size_t i = 0; i < n2; i++) {
    R[i] = a[mid + i];
  }

  L[n1] = INF;
  R[n2] = INF;
  ll i = 0;
  ll j = 0;
  ll r = 0;
  for (size_t k = left; k < right; k++) {
    if (L[i] <= R[j]) {
      a[k] = L[i];
      i++;
    } else {
      a[k] = R[j];
      r += n1 - i;
      j++;
    }
  }
  return r;
}
ll merge2(pair<int, char> *a, int left, int mid, int right) {
  ll n1 = mid - left;
  ll n2 = right - mid;
  vector<pair<int, char>> L(n1 + 1);
  vector<pair<int, char>> R(n2 + 1);
  for (size_t i = 0; i < n1; i++) {
    L[i] = a[left + i];
  }
  for (size_t i = 0; i < n2; i++) {
    R[i] = a[mid + i];
  }

  L[n1] = make_pair(INF, ' ');
  R[n2] = make_pair(INF, ' ');
  ll i = 0;
  ll j = 0;
  ll r = 0;
  for (size_t k = left; k < right; k++) {
    if (L[i].first <= R[j].first) {
      a[k] = L[i];
      i++;
    } else {
      a[k] = R[j];
      r += n1 - i;
      j++;
    }
  }
  return r;
}
ll mergeSort2(pair<int, char> *a, int left, int right) {
  ll res = 0;
  if (left + 1 < right) {
    int mid = (left + right) / 2;
    res = mergeSort2(a, left, mid);
    res += mergeSort2(a, mid, right);
    res += merge2(a, left, mid, right);
  }
  return res;
}
ll mergeSort(ll *a, int left, int right) {
  ll res = 0;
  if (left + 1 < right) {
    int mid = (left + right) / 2;
    res = mergeSort(a, left, mid);
    res += mergeSort(a, mid, right);
    res += merge(a, left, mid, right);
  }
  return res;
}
int partition(pair<int, char> *a, int p, int r) {
  pair<int, char> x = a[r];
  int i = p - 1;
  for (size_t j = p; j < r; j++) {
    if (a[j].first <= x.first) {
      i++;
      swap(a[i], a[j]);
    }
  }
  swap(a[i + 1], a[r]);
  return i + 1;
}
void quick(pair<int, char> *a, int p, int r) {
  if (p < r) {
    int q = partition(a, p, r);
    quick(a, p, q - 1);
    quick(a, q + 1, r);
  }
}

ll n;
int ci = 0;
ll P[1000010];
struct Node {
  int key;
  int priority;
  Node *parent, *left, *right;
  Node(int key, int priority);
  Node() {}
};
Node NIL;
Node::Node(int key, int priority) : key(key), priority(priority) {
  left = &NIL;
  right = &NIL;
}
Node *root = new Node();
void cenrec(Node *k) {
  if (k->key == NIL.key)
    return;
  cenrec(k->left);
  cout << " " << k->key;
  cenrec(k->right);
}
void fastrec(Node *k) {
  if (k->key == NIL.key)
    return;
  cout << " " << k->key;
  fastrec(k->left);
  fastrec(k->right);
}
void insert(Node *v) {
  Node *y = &NIL;
  Node *x = root;
  while (x->key != NIL.key) {
    y = x;
    if (v->key < x->key) {
      x = x->left;
    } else {
      x = x->right;
    }
  }
  v->parent = y;
  if (y->key == NIL.key) {
    root = v;
  } else if (v->key < y->key) {
    y->left = v;
  } else {
    y->right = v;
  }
}

Node *find(Node *k, ll v) {
  if (k->key == NIL.key)
    return &NIL;
  if (k->key == v)
    return k;
  if (v < k->key)
    return find(k->left, v);
  return find(k->right, v);
}
void delp12(Node *x) {
  if (x->key == NIL.key)
    return;
  Node *l = x->left;
  Node *r = x->right;
  Node *pr = x->parent;

  if (l->key == NIL.key && r->key == NIL.key) {
    if (pr->left == x) {
      pr->left = &NIL;
    } else
      pr->right = &NIL;
  } else if (l->key != NIL.key) {
    if (pr->left == x) {
      pr->left = l;
    } else
      pr->right = l;
    l->parent = pr;
  } else if (r->key != NIL.key) {
    if (pr->left == x) {
      pr->left = r;
    } else
      pr->right = r;
    r->parent = pr;
  }
}
Node *get_next(Node *k) {
  if (k->key == NIL.key)
    return &NIL;
  Node *res = get_next(k->left);
  if (res->key != NIL.key)
    return res;
  return k;
}
void del(Node *x) {

  if (x->key == NIL.key)
    return;
  Node *l = x->left;
  Node *r = x->right;
  Node *pr = x->parent;

  if (l->key != NIL.key && r->key != NIL.key) {
    Node *nex = get_next(r);
    x->key = nex->key;
    delp12(nex);
  } else {
    delp12(x);
  }
}
Node *rightRotate(Node *t) {
  Node *s = t->left;
  t->left = s->right;
  s->right = t;
  return s;
}
Node *leftRotate(Node *t) {
  Node *s = t->right;
  t->right = s->left;
  s->left = t;
  return s;
}
Node *_insert(Node *t, int key, int priority) {
  if (t->key == NIL.key) {
    return new Node(key, priority);
  }
  if (key == t->key) {
    return t;
  }

  if (key < t->key) {
    t->left = _insert(t->left, key, priority);
    if (t->priority < t->left->priority) {
      t = rightRotate(t);
    }
  } else {
    t->right = _insert(t->right, key, priority);
    if (t->priority < t->right->priority) {
      t = leftRotate(t);
    }
  }
  return t;
}
Node *delete1(Node *t, int key);
Node *_delete(Node *t, int key) {
  if (t->left->key == NIL.key && t->right->key == NIL.key) {
    return &NIL;
  } else if (t->left->key == NIL.key) {
    t = leftRotate(t);
  } else if (t->right->key == NIL.key) {
    t = rightRotate(t);
  } else {
    if (t->left->priority > t->right->priority) {
      t = rightRotate(t);
    } else
      t = leftRotate(t);
  }
  return delete1(t, key);
}
Node *delete1(Node *t, int key) {
  if (t->key == NIL.key) {
    return &NIL;
  }
  if (key < t->key) {
    t->left = delete1(t->left, key);
  } else if (key > t->key) {
    t->right = delete1(t->right, key);
  } else
    return _delete(t, key);
  return t;
}
int H;
int left(int i) { return i * 2 + 1; }
int right(int i) { return i * 2 + 2; }
struct edge {
  int from, to;
  ll val;
  edge(int from, int to, ll val) : from(from), to(to), val(val) {}
};
ll k;
int _rank[1010];
int temp[1010];
bool compare_sa(int i, int j) {
  if (_rank[i] != _rank[j])
    return _rank[i] < _rank[j];
  else {
    int ri = i + k <= n ? _rank[i + k] : -1;
    int rj = j + k <= n ? _rank[j + k] : -1;
    return ri < rj;
  }
}
void construct_sa(string S, int *sa) {
  n = S.length();

  for (size_t i = 0; i <= n; i++) {
    sa[i] = i;
    _rank[i] = i < n ? S[i] : -1;
  }

  for (k = 1; k <= n; k *= 2) {
    sort(sa, sa + n + 1, compare_sa);

    // saはソート後の接尾辞の並びになっている、rankは元の位置のindexを保持したまま、更新されている。
    // ピンとこなかった部分
    temp[sa[0]] = 0;
    for (size_t i = 1; i <= n; i++) {
      temp[sa[i]] = temp[sa[i - 1]] + (compare_sa(sa[i - 1], sa[i]) ? 1 : 0);
    }
    for (size_t i = 0; i <= n; i++) {
      _rank[i] = temp[i];
    }
  }
}
bool contain(string S, int *sa, string T) {
  int a = 0, b = S.length();
  // sa は 接尾辞が辞書順に並んでいる、入っているのはその位置のインデックス
  while (b - a > 1) {
    int c = (a + b) / 2;
    if (S.compare(sa[c], T.length(), T) < 0)
      a = c;
    else
      b = c;
  }
  return S.compare(sa[b], T.length(), T) == 0;
}

#define bit(x, v) ((ll)x << v)

class BIT {

  static const int MAX_N = 500010;

public:
  BIT() { memset(bit, 0, sizeof(bit)); }
  ll bit[MAX_N + 1], n;
  ll sum(int i) {
    ll s = 0;
    while (i > 0) {
      s += bit[i];
      i -= i & -i;
    }
    return s;
  }

  void add(int i, int x) {
    while (i <= n) {
      bit[i] += x;
      i += i & -i;
    }
  }
};
struct UnionFind {
  vector<int> A;
  UnionFind(int n) : A(n, -1) {}
  int find(int x) {
    if (A[x] < 0)
      return x;
    return A[x] = find(A[x]);
  }
  void unite(int x, int y) {
    x = find(x), y = find(y);
    if (x == y)
      return;
    if (A[x] > A[y])
      swap(x, y);
    A[x] += A[y];
    A[y] = x;
  }
  int ngroups() {
    int ans = 0;
    for (auto a : A)
      if (a < 0)
        ans++;
    return ans;
  }
};
void yes() {
  cout << "Yes\\n";
  exit(0);
}
void no() {
  cout << "No\\n";
  exit(0);
}
vector<ll> getp(ll n) {

  vector<ll> res;
  ll a = 2;
  if (n % 2 == 0) {
    res.push_back(2);
    while (n % 2 == 0)
      n /= 2;
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {
      res.push_back(i);
      while (n % i == 0)
        n /= i;
    }
  }
  if (n != 1)
    res.push_back(n);
  return res;
}
vector<ll> getp2(ll n) {

  vector<ll> res;
  ll a = 2;
  if (n % 2 == 0) {

    while (n % 2 == 0) {
      n /= 2;
      res.push_back(2);
    }
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {

      while (n % i == 0) {
        n /= i;
        res.push_back(i);
      }
    }
  }
  if (n != 1)
    res.push_back(n);
  return res;
}
vector<pll> getp3(ll n) {
  vector<pll> res;
  ll a = 2;
  int si = 0;
  if (n % 2 == 0) {

    res.push_back(make_pair(2, 0));
    while (n % 2 == 0) {
      n /= 2;
      res[si].second++;
    }
    si++;
  }

  for (ll i = 3; i * i <= n; i += 2) {
    if (n % i == 0) {
      res.push_back(make_pair(i, 0));
      while (n % i == 0) {
        n /= i;
        res[si].second++;
      }
      si++;
    }
  }
  if (n != 1) {
    res.push_back(make_pair(n, 1));
  }
  return res;
}

vector<ll> getDivisors(ll n) {

  vector<ll> res;
  ll a = 2;
  res.push_back(1);
  for (ll i = 2; i * i <= n; i++) {

    if (n % i == 0) {
      res.push_back(i);
      if (n / i != i)
        res.push_back(n / i);
    }
  }
  return res;
}

struct ve {
public:
  vector<ve> child;
  int _t = INF;
  ve(int t) : _t(t) {}
  ve(ve _left, ve _right) {
    _t = _left._t + _right._t;
    child.push_back(_left);
    child.push_back(_right);
  }
  bool operator<(const ve &t) const { return _t > t._t; }
};

vector<bool> elas(ll n) {
  vector<bool> r(n);
  fill(r.begin(), r.end(), 1);
  r[0] = 0;
  r[1] = 0;
  for (ll i = 2; i * i < n; i++) {
    if (!r[i])
      continue;
    ll ti = i * 2;
    while (ti < n) {
      r[ti] = false;
      ti += i;
    }
  }
  return r;
}
bool isPrime(ll v) {
  for (ll i = 2; i * i <= v; i++) {
    if (v % i == 0)
      return false;
  }
  return true;
}

class SegTree {

public:
  const static int MAX_N = 100010;
  const static int DAT_SIZE = (1 << 18) - 1;
  int N, Q;
  int A[MAX_N];

  ll data[DAT_SIZE], datb[DAT_SIZE];
  void init(int _n) {
    N = 1;
    while (N < _n)
      N <<= 1;
    memset(data, 0, sizeof(data));
    memset(datb, 0, sizeof(datb));
  }
  void init(int _n, ll iv) {
    N = 1;
    while (N < _n)
      N <<= 1;
    rep(i, DAT_SIZE) {
      data[i] = iv;
      datb[i] = iv;
    }
  }
  void add(int a, int b, int x) { add(a, b + 1, x, 0, 0, N); }
  void add(int a, int b, int x, int k, int l, int r) {
    if (a <= l && r <= b) {
      data[k] += x;
    } else if (l < b && a < r) {
      datb[k] += (min(b, r) - max(a, l)) * x;
      add(a, b, x, k * 2 + 1, l, (l + r) / 2);
      add(a, b, x, k * 2 + 2, (l + r) / 2, r);
    }
  }

  void change(int a, int b, int x) { change(a, b + 1, x, 0, 0, N); }
  void change(int a, int b, int x, int k, int l, int r) {
    if (a <= l && r <= b) {
      data[k] = x;
    } else if (l < b && a < r) {
      datb[k] = x;
      change(a, b, x, k * 2 + 1, l, (l + r) / 2);
      change(a, b, x, k * 2 + 2, (l + r) / 2, r);
    }
  }

  ll sum(int a, int b) { return sum(a, b + 1, 0, 0, N); }
  ll sum(int a, int b, int k, int l, int r) {
    if (b <= l || r <= a) {
      return 0;
    }
    if (a <= l && r <= b) {
      return data[k] * (r - l) + datb[k];
    }

    ll res = (min(b, r) - max(a, l)) * data[k];
    res += sum(a, b, k * 2 + 1, l, (l + r) / 2);
    res += sum(a, b, k * 2 + 2, (l + r) / 2, r);
    return res;
  }
};
class Segment;
class Circle;

class Point {
public:
  double x, y;

  Point(double x = 0, double y = 0) : x(x), y(y) {}

  Point operator+(Point p) { return Point(x + p.x, y + p.y); }
  Point operator-(Point p) { return Point(x - p.x, y - p.y); }
  Point operator*(double a) { return Point(a * x, a * y); }
  Point operator/(double a) { return Point(x / a, y / a); }

  double abs() { return sqrt(norm()); }
  double norm() { return x * x + y * y; }

  bool operator<(const Point &p) const { return x != p.x ? x < p.x : y < p.y; }
  bool operator==(const Point &p) const {
    return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
  }
  static double dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }
  static double cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }
  static bool isOrthogonal(Point a, Point b) { return EQ(dot(a, b), 0.0); }
  static bool isOrthogonal(Point a1, Point a2, Point b1, Point b2) {
    return isOrthogonal(a1 - a2, b1 - b2);
  }
  static bool isOrthogonal(Segment s1, Segment s2);

  static bool isPalallel(Point a, Point b) { return EQ(cross(a, b), 0.0); }
  static bool isPalallel(Point a1, Point a2, Point b1, Point b2) {
    return isPalallel(a1 - a2, b1 - b2);
  }
  static bool isPalallel(Segment s1, Segment s2);

  static const int COUNTER_CLOCKWISE = 1;
  static const int CLOCKWISE = -1;
  static const int ONLINE_BACK = 2;
  static const int ONLINE_FRONT = -2;
  static const int ON_SEGMENT = 0;
  static int ccw(Point p0, Point p1, Point p2) {
    Point a = p1 - p0;
    Point b = p2 - p0;
    if (cross(a, b) > EPS)
      return COUNTER_CLOCKWISE;
    if (cross(a, b) < -EPS)
      return CLOCKWISE;
    if (dot(a, b) < -EPS)
      return ONLINE_BACK;
    if (a.norm() < b.norm())
      return ONLINE_FRONT;
    return ON_SEGMENT;
  }

  static bool intersect(Point p1, Point p2, Point p3, Point p4) {
    return (ccw(p1, p2, p3) * ccw(p1, p2, p4) <= 0 &&
            ccw(p3, p4, p1) * ccw(p3, p4, p2) <= 0);
  }
  static bool intersect(Segment s1, Segment s2);
  static Point project(Segment s, Point p);

  static Point reflect(Segment s, Point p);

  static Point getDistance(Point a, Point b) { return (a - b).abs(); }

  static double getDistanceLP(Segment s, Point p);

  static double getDistanceSP(Segment s, Point p);

  static double getDistance(Segment s1, Segment s2);

  static Point getIntersection(Segment s1, Segment s2);

  static pair<Point, Point> crossPoints(Circle c, Segment s);

  static int contains(vector<Point> g, Point p) {
    int n = g.size();
    bool x = false;
    rep(i, n) {
      Point a = g[i] - p, b = g[(i + 1) % n] - p;
      // 線の上に載っているか
      if (std::abs(cross(a, b)) < EPS && dot(a, b) < EPS)
        return 1;

      // pを基準として上下にあるか
      // または外積が正か?(→にあるか)
      if (a.y > b.y)
        swap(a, b);
      if (a.y < EPS && EPS < b.y && cross(a, b) > EPS)
        x = !x;
    }
    return x ? 2 : 0;
  }

  static vector<Point> andrewScan(vector<Point> s) {
    vector<Point> u, l;
    if (s.size() < 3)
      return s;
    sort(all(s));
    u.push_back(s[0]);
    u.push_back(s[1]);
    l.push_back(s[s.size() - 1]);
    l.push_back(s[s.size() - 2]);

    for (int i = 2; i < s.size(); i++) {

      for (int _n = u.size();
           _n >= 2 && ccw(u[_n - 2], u[_n - 1], s[i]) > CLOCKWISE; _n--) {
        u.pop_back();
      }
      u.push_back(s[i]);
    }

    for (int i = s.size() - 3; i >= 0; i--) {

      for (int _n = l.size();
           _n >= 2 && ccw(l[_n - 2], l[_n - 1], s[i]) > CLOCKWISE; _n--) {
        l.pop_back();
      }
      l.push_back(s[i]);
    }

    reverse(all(l));
    for (int i = u.size() - 2; i >= 1; i--) {
      l.push_back(u[i]);
    }

    return l;
  }
  void get_cin() { cin >> x >> y; }
};

class Segment {
public:
  Point p1, p2;
  Segment() {}
  Segment(Point p1, Point p2) : p1(p1), p2(p2) {}
  void get_cin() { cin >> p1.x >> p1.y >> p2.x >> p2.y; }
  Point p1tp2() { return p2 - p1; }
  Point p2tp1() { return p1 - p2; }
  double abs() { return std::abs(norm()); }
  double norm() { return (p2 - p1).norm(); }
};

bool Point::isOrthogonal(Segment s1, Segment s2) {
  return EQ(dot(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
bool Point::isPalallel(Segment s1, Segment s2) {
  return EQ(cross(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
bool Point::intersect(Segment s1, Segment s2) {
  return intersect(s1.p1, s1.p2, s2.p1, s2.p2);
}
Point Point::project(Segment s, Point p) {
  Point base = s.p2 - s.p1;
  double r = Point::dot(p - s.p1, base) / base.norm();
  return s.p1 + base * r;
}
Point Point::reflect(Segment s, Point p) { return (project(s, p) * 2) - p; }
double Point::getDistanceLP(Segment s, Point p) {
  return std::abs(cross(s.p2 - s.p1, p - s.p1) / (s.p2 - s.p1).abs());
}
double Point::getDistanceSP(Segment s, Point p) {
  if (dot(s.p2 - s.p1, p - s.p1) < 0.0)
    return (p - s.p1).abs();
  if (dot(s.p1 - s.p2, p - s.p2) < 0.0)
    return (p - s.p2).abs();
  return getDistanceLP(s, p);
}
double Point::getDistance(Segment s1, Segment s2) {
  if (intersect(s1, s2))
    return 0.0;
  return min({getDistanceSP(s1, s2.p1), getDistanceSP(s1, s2.p2),
              getDistanceSP(s2, s1.p1), getDistanceSP(s2, s1.p2)});
}

Point Point::getIntersection(Segment s1, Segment s2) {
  // (s1.p1 - s2.p1).norm()
  auto bs = s1.p2 - s1.p1;
  auto n1 = s2.p1 - s1.p1;
  auto n2 = s2.p2 - s1.p1;
  auto c1 = std::abs(cross(n1, bs)) / bs.norm();
  auto c2 = std::abs(cross(n2, bs)) / bs.norm();
  return s2.p1 + (s2.p2 - s2.p1) * (c1 / (c1 + c2));
  // c1:c2=t:1-t
  // c2t=(1-t)c1
  // t/(1-t)=c1/(c1+c2)
  //
}

double arg(Point p) { return atan2(p.y, p.x); }
Point polar(double a, double r) { return Point(cos(r) * a, sin(r) * a); }
class Circle {
public:
  Point c;
  double r;
  Circle(Point c = Point(), double r = 0.0) : c(c), r(r) {}
  void get_cin() { cin >> c.x >> c.y >> r; }
  static pair<Point, Point> getCrossPoints(Circle c1, Circle c2) {
    double d = (c1.c - c2.c).abs(); // 中心点どうしの距離
    double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d));
    double t = arg(c2.c - c1.c);
    return make_pair(c1.c + polar(c1.r, t + a), c1.c + polar(c1.r, t - a));
  }
};

pair<Point, Point> Point::crossPoints(Circle c, Segment s) {
  auto pp = project(s, c.c);
  auto f = (pp - c.c).norm();
  auto mu = sqrt(c.r * c.r - f);
  auto e = s.p1tp2() / s.p1tp2().abs();
  return make_pair(pp + e * mu, pp - e * mu);
}

ll divRm(string s, ll x) {

  ll r = 0;
  for (ll i = 0; i < s.size(); i++) {
    r *= 10;
    r += s[i] - '0';
    r %= x;
  }
  return r;
}
ll cmbi(ll x, ll b) {

  ll res = 1;
  for (size_t i = 0; i < b; i++) {
    res *= x - i;
    res %= INF;
    res *= inv[b - i];
    res %= INF;
  }
  return res;
}

double digsum(ll x) {
  ll res = 0;
  while (x > 0) {
    res += x % 10;
    x /= 10;
  }
  return res;
}
bool check_parindrome(string s) {
  int n = s.size();
  rep(i, n / 2) {
    if (s[i] != s[n - i - 1]) {
      return false;
    }
  }
  return true;
}

void solv() {
  cin >> n;
  ll a[200010];

  rep(i, n) cin >> a[i];
  ll aia[400010];
  all0(aia);
  ll hf = 200000;
  rep(i, n) {
    if (a[i] > n - 1)
      continue;
    aia[a[i] + i + hf]++;
  }
  ll res = 0;
  rep(i, n) {
    if (a[i] > n - 1)
      continue;
    res += aia[i - a[i] + hf];
    if (a[i] + hf + i == i + a[i] - hf)
      res--;
  }
  cout << res << endl;
  // ll x;
  // cin >> x;
  // map<ll, ll> mp;
  // rep2(i, -100000, 10001) {
  //	mp[pow(i, 5)] = i;
  // }
  // for(ll b = -100000; b <= 100000; b++) {
  //	ll t = x + pow(b,5);
  //	if (mp.count(t) > 0) {
  //		cout << mp[t] << " " << b << endl;
  //		return;
  //	}
  // }
}
int main() {
  // COMinit();
  solv();
  return 0;
}