#include <bits/stdc++.h>
#define NMAX 9005
#define MOD 666013
#define INF 0x3f3f3f3f
#define x first
#define y second
#define pb push_back

using namespace std;

ifstream fin("medie.in");
ofstream fout("medie.out");

int fr[NMAX];
int v[NMAX];

int main() {
	int n,i,j,ans=0,x;

	fin>>n;
	for(i=1;i<=n;++i) {
		fin>>v[i];
		++fr[v[i]];
	}

	for(i=1;i<=n;++i)
		for(j=i+1;j<=n;++j)
			if(((v[i]+v[j])&1)==0) {
				ans+=fr[(v[i]+v[j])/2];

				if((v[i]+v[j])/2==v[i]) --ans;
				if((v[i]+v[j])/2==v[j]) --ans;
			}

	fout<<ans;

	return 0;
}