#include <stdio.h>

int n,a[9010],b[8000],f[8000],aux,s;

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d",&n);
	int i,j;
	for (i=1; i<=n; ++i) scanf("%d",&a[i]);
	for (i=1; i<n; ++i)
	{
		++f[a[i]];
		for (j=i+1; j<=n; ++j)
		{
			aux=(a[i]+a[j])>>1;
			if (aux<<1==a[i]+a[j]) ++b[aux];
		}
	}
	++f[a[n]];
	s=0;
	for (i=1; i<=n; ++i)
	{
		if (b[a[i]]!=0) s+=b[a[i]]-f[a[i]]+1;
	}
	printf("%d\n",s);
	return 0;
}