#include<stdio.h>
int car[7005], v[9005];
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n, i, j, a;
	long long s=0;
	double x;
	scanf("%d", &n);
	for (i=1; i<=n; i++){
		scanf("%d", &v[i]);
		car[v[i]]++;
	}
	for (i=1; i<n; i++)
		for (j=i+1; j<=n; j++){
			x = (v[i]+v[j]) * 0.5;
			a = x;
			if (x==a){
				s+=car[a];
				if (a==v[i]){
					if (a==v[j])
						s-=2;
					else
						s-=1;
				}
			}
		}
	printf("%lld\n",s);
	fclose(stdin);
	fclose(stdout);
	return 0;
}