#include <bits/stdc++.h>

#define MAX_N 9000
#define MAX_VAL 7000

using namespace std;

int v[MAX_N], f[2 * MAX_VAL + 1];

int main() {
    ifstream cin( "medie.in" );
    ofstream cout( "medie.out" );

    int n, sol, i, j;

    cin >> n;
    for ( i = 0; i < n; i++ ) {
        cin >> v[i];
        f[v[i] * 2]++;
    }

    sol = 0;
    for ( i = 0; i < n; i++ ) {
        for ( j = i + 1; j < n; j++ )
            sol += max( f[v[i] + v[j]] - (v[i] == v[j]) * 2, 0 );
    }

    cout << sol;

    return 0;
}