#include <fstream>
#define f in
#define g out

using namespace std;
ifstream in ( "medie.in" );
ofstream out( "medie.out" );
short int v[10000], fr1[10000], fr2[10000];
int i, n, j, 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;
}