#include <fstream>]

using namespace std;
ifstream f ( "medie.in" );
ofstream g( "medie.out" );
int v[9001], fr1[7001], fr2[7001], i, n, j;
long long sol;

int main() {
    f>>n;
    for ( i=1; i <= n; i++ ){
        f>>v[i];
        fr1[v[i]]++;
        for ( j=1; j < i; j++ )
            if ( ( v[i] + v[j] ) % 2 == 0 )
                fr2[ ( v[i] + v[j] ) / 2 ]++;
    }
    for ( i=1; i <= n; i++ )
        sol += fr2[v[i]] - fr1[v[i]] + 1;
    g<<sol;
    return 0;
}