#include<bits/stdc++.h>
using namespace std;
int n,ans;
int a[110];
map<int,int> m;
int main(){
	cin>>n;
	for(int i=1;i<=n;i++) cin>>a[i];
	for(int i=1;i<=n;i++){
		for(int j=i+1;j<=n;j++){
			m[a[i]+a[j]]++;
		}
	}
	for(int i=1;i<=n;i++) ans+=m[a[i]];
	cout<<ans;
	return 0;
}
/**************************************************************
	Problem: 2330
	User: huangsheng
	Language: C++
	Result: Wrong Answer
****************************************************************/