#include <fstream>
#include <iostream>
using namespace std;
int a[9005];
int freq[7005];
int main(){
	ifstream f("medie.in");
	ofstream h("medie.out");
	int i,j,rez=0,x,n;
	f>>n;
	for(i=1;i<=n;++i)
		f>>a[i];
	for(i=1;i<=n;++i)++freq[a[i]];
	for(i=2;i<=n;++i)
		for(j=1;j<i;++j)
			if((a[i]+a[j])%2==0){
				x=(a[i]+a[j])/2;
				rez+=freq[x];//de cate ori apare aceasta medie in sir
				if(a[i]==a[j])
					rez-=2;
			}
	h<<rez;
	return 0;
}