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

int main() {
    int N; cin >> N;
    int M = 200'000;
    int A[N+1];
    for (int i = 1; i <= N; i++) cin >> A[i];
    
    vector<vector<int>> C(M+1);
    for (int i = 1; i <= N; i++) {
        int d = 1;
        while(A[i] - d * i >= -M) {
            C[d].push_back(A[i] - d * i);
            d++;
        }
    }
    
    long long ans = 0;
    
    for (int d = 1; d <= M; d++) {
        map<int, long long> cnt;
        for (int x : C[d]) {
        	ans += cnt[-x];
        	cnt[x]++;
        }
    }
    
    cout << ans << endl;
}