#include<fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
int v[9001],nr[7001],i,n,med,j;
long long SUM;
int main ()
{
	f>>n;
	for(i=1;i<=n;++i)
	{
		f>>v[i];
		nr[v[i]]++;
	}
	for(i=1;i<=n;++i)
		for(j=i+1;j<=n;++j)
		{
			med=v[i]+v[j];
			if(med%2)
				continue;
			med/=2;
			if(v[i]!=v[j])
				SUM+=nr[med];
			else
				SUM+=nr[med]-2;
		}
	g<<SUM;
	return 0;
}