#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,a[5005] = {},ans = 0,max_ = -1;
	cin>>n;
	for(int i = 0;i<n;i++){
		int t;
		cin>>t;
		a[t]++;
		max_ = max(max_,t);
	}
	for(int i = 1;i<=max_;i++){
		if(a[i]<2){
			continue;
		}
		for(int j = 1;j*2<=i;j++){
			if(a[j]>0&&a[i-j]>0){
				if(j==i-j&&a[j]<2){
					continue;
				}
				ans++;
			}
		}
	}
	cout<<ans;
	return 0;
}