#include<fstream>
using namespace std;
int n,v[9010],nr[7010];
long long sol;

void Citire()
{
	int i;
	ifstream fin("medie.in");
	fin>>n;
	for(i=1;i<=n;i++)
	{
		fin>>v[i];
		nr[v[i]]++;
	}
	fin.close();
}

void Rezolvare()
{
	int i,j;
	for(i=1;i<n;i++)
	{
		for(j=i+1;j<=n;j++)
		{
			if((v[i]+v[j])%2==0)
			{
				if(v[i]!=v[j])
					sol+=(long long)nr[(v[i]+v[j])/2];
				else
					sol+=(long long)nr[(v[i]+v[j])/2]-2;
			}
		}
	}
}

void Afisare()
{
	ofstream fout("medie.out");
	fout<<sol<<"\n";
	fout.close();
}

int main()
{
	Citire();
	Rezolvare();
	Afisare();
	return 0;
}