Problem:
Score: 500 points

Problem Statement

You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens.

There are N attendees in the party, and they are given attendee numbers from 1 through N. The height of Attendee i is A_i.

According to an examination beforehand, you know that a pair of attendees satisfying the condition below will make the transaction.

The absolute difference of their attendee numbers is equal to the sum of their heights.

There are \frac{N(N-1)}{2} ways to choose two from the N attendees and make a pair. Among them, how many satisfy the condition above?

P.S.: We cannot let you know the secret.

Constraints

All values in input are integers.

2 \leq N \leq 2 \times 10^5

1 \leq A_i \leq 10^9\ (1 \leq i \leq N)

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 \dots A_N

Output

Print the number of pairs satisfying the condition.

Sample Input 1

6
2 3 3 1 3 1

Sample Output 1

3

A_1 + A_4 = 3, so the pair of Attendee 1 and 4 satisfy the condition.

A_2 + A_6 = 4, so the pair of Attendee 2 and 6 satisfy the condition.

A_4 + A_6 = 2, so the pair of Attendee 4 and 6 satisfy the condition.

No other pair satisfies the condition, so you should print 3.

Sample Input 2

6
5 2 4 2 8 8

Sample Output 2

0

No pair satisfies the condition, so you should print 0.

Sample Input 3

32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5

Sample Output 3

22

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

#define MEM(a, b) memset(a, (b), sizeof(a))
#define repn(i, a, n) for (int i = a; i < n; i++)
#define revn(i, a, n) for (int i = a; i > n; i--)
#define rep(i, n) for (int i = 0; i < n; i++)
#define rev(i, a) for (int i = a; i > -1; i--)
#define mp make_pair
#define pb push_back
#define INF 1000000010
#define LLINF 1000000000000000010;
#define MOD 1000000007 // 998244353
#define PI 3.141592653589793238462643383279502884197169399375105820974945
#define fastio ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define all(v) v.begin(), v.end()
typedef long long ll;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<ll> vll;
typedef pair<int, int> pii;
typedef vector<string> vs;
typedef vector<vi> vii;

//debugging output in cerr
#define n_l '\n'
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: "; cerr << to_string(__VA_ARGS__) << endl
template <typename T, size_t N> int SIZE(const T (&t)[N]){ return N; } template<typename T> int SIZE(const T &t){ return t.size(); } string to_string(const string s, int x1=0, int x2=1e9){ return '"' + ((x1 < s.size()) ? s.substr(x1, x2-x1+1) : "") + '"'; } string to_string(const char* s) { return to_string((string) s); } string to_string(const bool b) { return (b ? "true" : "false"); } string to_string(const char c){ return string({c}); } template<size_t N> string to_string(const bitset<N> &b, int x1=0, int x2=1e9){ string t = ""; for(int __iii__ = min(x1,SIZE(b)),  __jjj__ = min(x2, SIZE(b)-1); __iii__ <= __jjj__; ++__iii__){ t += b[__iii__] + '0'; } return '"' + t + '"'; } template <typename A, typename... C> string to_string(const A (&v), int x1=0, int x2=1e9, C... coords); int l_v_l_v_l = 0, t_a_b_s = 0; template <typename A, typename B> string to_string(const pair<A, B> &p) { l_v_l_v_l++; string res = "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; l_v_l_v_l--; return res; } template <typename A, typename... C> string to_string(const A (&v), int x1, int x2, C... coords) { int rnk = rank<A>::value; string tab(t_a_b_s, ' '); string res = ""; bool first = true; if(l_v_l_v_l == 0) res += n_l; res += tab + "["; x1 = min(x1, SIZE(v)), x2 = min(x2, SIZE(v)); auto l = begin(v); advance(l, x1); auto r = l; advance(r, (x2-x1) + (x2 < SIZE(v))); for (auto e = l; e != r; e = next(e)) { if (!first) { res += ", "; } first = false; l_v_l_v_l++; if(e != l){ if(rnk > 1) { res += n_l; t_a_b_s = l_v_l_v_l; }; } else{ t_a_b_s = 0; } res += to_string(*e, coords...); l_v_l_v_l--; } res += "]"; if(l_v_l_v_l == 0) res += n_l; return res; } void debugm(){;} template<typename Heads, typename... Tails> void debugm(Heads H, Tails... T){ cerr << to_string(H) << " | "; debugm(T...); } 
#define debugm(...) cerr << "[" << #__VA_ARGS__ << "]: "; debugm(__VA_ARGS__); cerr << endl

double round(double num, int dec) {return std::round(num*pow(10,dec))/pow(10,dec);}
vector<string> split(string s, char delim) {replace(s.begin(),s.end(),delim,' ');stringstream ss(s); string buf;vector<string> v;while (ss>>buf) v.push_back(buf);return v;}
template<typename T, int size> int siz(T(&)[size]){return size;}
long long POW(long long base, long long exp) {long long res = 1;while (exp > 0) {if (exp % 2 == 1){res= (res * base) % MOD;}exp = exp >> 1;base = (base * base) % MOD;}return res;}




int main() {
    int n; cin>>n;
    vi a(n+1);
    map<int, int> m;
    ll ans = 0;
    repn(i, 1, n+1) {
        cin >> a[i];
        ans += m[i-a[i]];
        // debug(m);
        ++m[a[i]+i];
    }
    cout << ans;
}