#include<cstdio>
using namespace std;
int n,a[10001],frecv[10001],i,j,x,nr;
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d\n",&n);
	for(i=1;i<=n;i++) scanf("%d\n",&a[i]),frecv[a[i]]++;
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
		{
			x=a[j]+a[i];
			if((x&1)==0)
			{
				x=(a[j]+a[i])>>1;
				nr+=frecv[x];
				if(a[i]==x) nr-=2;
			}
		}
	printf("%d\n",nr);
	return 0;
}