#include<cstdio>
void read(),solve();
int a[9010],V[7010],i,j,n,SOL;
int main()
{
	read();
	solve();
	return 0;
}
void read()
{
	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]];}
}
void solve()
{
	for(i=1;i<=n;i++)
		for(j=i+1;j<=n;j++)
		{
			if(!((a[i]+a[j])%2))SOL+=V[(a[i]+a[j])/2];
			if(a[i]==a[j])SOL-=2;
		}
	printf("%d",SOL);
}