#include<fstream>
using namespace std;

unsigned int n,i,j,a[9001], p[7001],sum;
int main()
{
	ifstream f("medie.in");
	ofstream g("medie.out");
	f>>n;
	for(i=1;i<=n;i++)
	{
		f>>a[i];
		for(j=i-1;j>0;j--)
			if((a[i]+a[j])%2==0)
				p[(a[i]+a[j])/2]++;
	}
	for(i=1;i<=n;i++)
	{
		//g<<p[i]<<" ";
		sum+=p[a[i]];
	}
	g<<sum;
	return 0;
}