#include<cstdio>
int v[6005], n, i, cit, min = 60001, max, a, j;
long long s;
double x;
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d", &n);
	for (i=1; i<=n; i++){
		scanf("%d", &cit);
		if (cit < min)
			min = cit;
		if (cit > max)
			max = cit;
		v[cit]++;
	}
	for (i=min; i<=max; i++){
		if (v[i] > 1)
			s+=v[i]*(v[i]-1)*(v[i]-2)/2;
		for (j=i+1; j<=max; j++){
			x = (i+j)*0.5;
			a = x;
			if (x-a == 0)
				s+=v[i]*v[j]*v[a];
		}
	}
	printf("%lld\n", s);
	fclose(stdin);
	fclose(stdout);
	return 0;
}