#include <fstream>
#include <cstdio>
using namespace std;

ifstream f("medie.in");
ofstream g("medie.out");
const int n = 7000;

int fr[n+1],v[n+2],x,maxim;
long long rez;

int main()
{
    int n;
    long long sol=0;
    f >> n ;
    for ( int i = 1 ; i <= n ; i ++ )
     {
        f >> v[i] ;
     fr[v[i]] ++ ;
     maxim=max(maxim,v[i]);
     }
    for ( int i = 1 ; i < maxim ; i ++ )
        for ( int j = i + 1 ; j <= maxim ; j ++ )
        if( ( v[i] + v[j] ) % 2 == 0 )
        {
                sol+=fr[(v[i]+v[j])/2];
                if( v[i] == v[j] )
                 sol -= 2 ;
        }
        g << sol ;
}