#include<cstdio>
#define DN 9005
using namespace std;
int  i,n,a[DN],v[2*DN],j,rez;
int main()
{
	freopen("medie.in", "r", stdin);
	freopen("medie.out", "w", stdout);
	scanf("%d", &n);
	for(i=1; i<=n; i++) 
	{
		scanf("%d", &a[i]);
		v[a[i]]++;
	}
	for(i=1; i<=n; i++)
		for(j=i+1; j<=n; j++)
			if((a[i]+a[j])%2==0) 
			{
				v[a[i]]--,v[a[j]]--;
				rez+=v[(a[i]+a[j])/2];
				v[a[i]]++,v[a[j]]++;
			}
	printf("%d", rez);
}