#include<iostream>
#include<stdio.h>
int a[9010],v[7010];
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n,i,j,X;
	long long nr=0;
	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)
			{
				X=(a[i]+a[j])>>1;
				nr+=v[X];
				if(a[i]==X) nr--;
				if(a[j]==X) nr--;
			}
	printf("%lld\n",nr);
	return 0;
}