// Problem : E - This Message Will Self-Destruct in 5s
// Contest : AtCoder - AtCoder Beginner Contest 166
// URL : https://atcoder.jp/contests/abc166/tasks/abc166_e
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define eb emplace_back
#define pb push_back
#define size(s) (int)s.size()
#define int long long
#define vi vector<int>
#define vs vector<string>
#define vv vector<vector<int>>
#define pii pair<int, int>
#define m_p(x, y) make_pair(x, y)
#define vp vector<pair<int, int>>
#define setbits(x) __builtin_popcountll(x)
#define f first
#define se second
#define inc(v, n, x) v.assign(n, x)
#define incd(v, n) v.resize(n)
#define iniz(n) memset(n, 0, sizeof(n))
#define inin(n) memset(n, -1, sizeof(n))
#define inimi(n) memset(n, 0xc0, sizeof(n))
#define inima(n) memset(n, 0x3f, sizeof(n))
#define all(v) (v).begin(), (v).end()
using namespace std;
using namespace __gnu_pbds;
template <typename T1, typename T2>
istream &operator>>(istream &is, vector<pair<T1, T2>> &v) {
  for (pair<T1, T2> &t : v) is >> t.f >> t.se;
  return is;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) is >> t;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  for (const T &t : v) {
    os << t << " ";
  }
  os << '\n';
  return os;
}
double pi = acos(-1.0);
int md = 1e9 + 7;
const int INF = 1e15;
int dx1[] = {0, 0, -1, 1};
int dy1[] = {1, -1, 0, 0};
template <class T>
T abst(T a) {
  return a < 0 ? -a : a;
}
template <class T>
using ordered_set = tree<T, null_type, less_equal<T>, rb_tree_tag,
                         tree_order_statistics_node_update>;
// find_by_order(k)  returns iterator to kth element starting from 0;
// order_of_key(k) returns count of elements strictly smaller than k;
struct HASH {
  size_t operator()(const pii &x) const {
    return (size_t)x.first * 37U + (size_t)x.second;
  }
};
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};

int Pow(int n, int x) {
  int ans = 1;
  while (x > 0) {
    if (x & 1) ans = (ans * n) % md;
    n = (n * n) % md;
    x = x >> 1;
  }
  return ans;
}

vi fact, inv;
void inverse(int n) {
  inv.resize(n + 1);
  inv[0] = 1;
  for (int i = 1; i <= n; i++) inv[i] = Pow(fact[i], md - 2);
}
void factorial(int n) {
  fact.resize(n + 1);
  fact[0] = 1;
  for (int i = 1; i <= n; i++) fact[i] = (fact[i - 1] * i) % md;
}
int ncr(int n, int r) { return (((fact[n] * inv[r]) % md) * inv[n - r]) % md; }
template <class T>
T max2(T a, T b) {
  return a > b ? a : b;
}
template <class T>
T min2(T a, T b) {
  return a < b ? a : b;
}

template <class T>
T maxm(initializer_list<T> l) {
  T ans = -INF;
  for (T i : l) ans = max2(ans, i);
  return ans;
}

void solve() {
  int n;
  cin >> n;
  vi v(n);
  cin >> v;
  int cnt = 0;
  map<int, int> mp1, mp2;
  for (int i = 0; i < n; i++) {
    cnt += mp1[i - v[i]];
    mp1[i + v[i]]++;
  }
  for (int i = n - 1; i >= 0; i--) {
    cnt += mp2[i + v[i]];
    mp2[i - v[i]]++;
  }
  cout << cnt / 2;
}

int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  // cin>>t;
  for (int i = 1; i <= t; i++) {
    solve();
  }
}


Measured memory_bytes: 50124