#include <cstdio>
using namespace std;

int n,a[9010],b[7010],f[7010],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]);
        ++f[a[i]];
        }
	for (i=1; i<n; ++i)
	{
		for (j=i+1; j<=n; ++j)
		{
//			aux=(a[i]+a[j])>>1;
			if (((a[i]+a[j])>>1)<<1==a[i]+a[j]) ++b[(a[i]+a[j])>>1];
		}
	}
	s=0;
	for (i=1; i<=n; ++i)
	{
		if (b[a[i]]) s+=b[a[i]]-f[a[i]]+1;
	}
	printf("%d\n",s);
	return 0;
}