#include <cstdio>
#include <iostream>
//#include <cstdlib>
//#include <windows.h>
//#include <Windowsx.h>
//#include <process.h>
//#include <winnt.h>
//#include <stddef.h>
//#include <fstream>
//#include <cmath>
//#include <iomanip>
//#include <cstring>
using namespace std;
/*
* Using Minecraft to play.
* It's your world.
* If you died,please play it again.
* Let's continue.
*/
int a[101];
int main(){
	int n , c , cnt=0;
	cin >> n;
	for(int i=1;i <= n;i++){
		cin >> a[i];
	}
	for(int i=1;i <= n;i++){
		for(int j=1;j <= n;j++){
			if(j == i){
				continue;
			}
			c = i + j;
			for(int k=1;k <= n;k++){
				if(a[k] == c){
					cnt++;
				}
			}
		}
	}
	cnt /= 2;
	cout << cnt;
	return 0;
}