#include <fstream>
using namespace std;
ifstream cin("medie.in");
ofstream cout("medie.out");
long long n ;
long long frecv [ 15001 ];
long long v [ 9001 ];
int main()
{
    cin >> n ;

    frecv [ 0 ]= 0 ;
    for ( int i = 1 ; i <= n ; i ++ )
    {
        cin >> v[ i ];
        frecv [v [ i ] ] ++ ;
    }

    int cont = 0 ;
    for ( int i = 1; i <= n ; i ++ )
    {
        frecv [ v [ i ] ] -- ;

        long long s = 2 * v [ i ];

        for( int j = 1; j <= n ; j ++ )
        {

            if ( i != j )
            {
                frecv [ v[j] ] -- ;

                if (s - v[ j ] > 0 &&  frecv [ s - v [ j ] ] > 0   )
                    cont += frecv [s - v[ j ]] ;




            }
        }

        for ( int j = 1; j <= n ; j ++ )
        {
            if ( i != j )
                frecv[ v[ j ]] ++ ;
        }



        frecv [ v[ i ] ] ++ ;
    }

    cout << cont ;

    cin.close();
    cout.close();
    return 0;
}