#include <bits/stdc++.h>
using namespace std;
int n,a[400],mp[20000],ans=0;
int main()
{
	//freopen("count.in","r",stdin);
	//freopen("count.out","w",stdout);
	cin>>n;
	for(int i=1;i<=n;i++) {cin>>a[i];mp[a[i]]=i;}
	for(int i=1;i<=n;i++)
		for(int j=i+1;j<=n;j++)
		{
			if(mp[a[i]+a[j]]>=1) 
				ans++;
		}	
	cout<<ans;
	return 0;
}

rt.