#include <fstream>
using namespace std;

ifstream F("medie.in");
ofstream G("medie.out");	

int A[9001],Nbr[7001],Frec[7001];
int s,N,i,j;

int main() 
{
	F>>N;
	
	for (i=1; i<=N; ++i) 
		F>>A[i], ++Nbr[A[i]];
	
	for (i=1; i<N; ++i)
		for (j=i+1; j<=N; ++j)
			if ( !((A[i]+A[j])%2) )
				Frec[ (A[i]+A[j])/2 ]++;
	
	for (i=1; i<=N; ++i)
		if (Frec[A[i]])
			s+=(Frec[A[i]]-Nbr[A[i]]+1);
	
	G<<s<<'\n';
}