#include<bits/stdc++.h>
using namespace std;
int n;
int a[101],b[200005],ans;
//1 1 1 1 1
//9 10
//先算出每两个数加起来的和
//放入桶中 
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
		b[a[i]]=1;
	}
	for(int i=1;i<=n;i++){
		for(int j=i+1;j<=n;j++){
			if(b[a[i]+a[j]]==1){
				b[a[i]+a[j]]=-1;
				ans++;
			}
		}
	}
	cout<<ans;
	return 0;
}
/**************************************************************
	Problem: 2330
	User: wuyunfeng
	Language: C++
	Result: Accepted
	Time:45 ms
	Memory:2856 kb
****************************************************************/