#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]]=1;
		}
	}
	for(int i=1;i<=n;i++) ans+=m[a[i]];
	cout<<ans;
	return 0;
}
/*
5 9
3 1 4 1 5
0

6 1
1 6 1 2 0 4
11
84
73264529

10 5 8 10 9 7 11
4  1        
6  4 0  6 4 1  5
*/



/**************************************************************
	Problem: 2330
	User: huangsheng
	Language: C++
	Result: Accepted
	Time:58 ms
	Memory:2340 kb
****************************************************************/