#include <cstdio>
double a[7005];
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int i,j,k,k2=0,n;
	scanf("%d",&n);
	for(i=1;i<=n;++i)
		scanf("%lf",&a[i]);
	for(i=1;i<=n;++i){
		for(j=1;j<=n;++j){
			for(k=1;k<=n;++k){
				if(i!=j&&i!=k&&j!=k&&(a[i]+a[j])/2==a[k])
					++k2;
			}
		}
	}
	printf("%d",k2/2);
}