#include <iostream>
#include <fstream>

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

int n,s,i,j,k,v[9005],f[70005];
long long rez;

int main() {
	
	fin>>n;
	for(i=1;i<=n;i++)
	{
		fin>>v[i];
		f[v[i]]++;
	}
	
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
		{
			s=v[i]+v[j];
			f[v[i]]--;	
			f[v[j]]--;
			if(s%2==0)	rez+=f[s/2];
			f[v[i]]++;	
			f[v[j]]++;
		}
	fout<<rez<<"\n";
}