#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000005
#define Inf64 1000000000000000001LL


int main(){
	
	int n;
	cin>>n;
	vector<long long> a(n);
	rep(i,n)cin>>a[i];
	
	long long ans = 0;
	for(long long i=2;i<=800;i++){
		for(long long j=1;j<i;j++){
			int ii = j-1,jj = (i-j)-1;
			if(jj>=n)continue;
			if(ii>=jj)break;
			if((a[ii]+a[jj])%i==0&&(a[ii]+a[jj])/i>800){
				//cout<<ii<<' '<<jj<<endl;
				ans++;
			}
		}
	}
	for(long long i=1;i<=800;i++){
		vector<long long> t(n);
		rep(j,n){
			t[j] = a[j] - (j+1) * i;
		}
		map<long long,int> mp;
		rep(j,n){
			ans += mp[-t[j]];
			mp[t[j]]++;
		}
	}
	cout<<ans<<endl;
}