// such hard problem :///
#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("medie.in");
ofstream fout("medie.out");

const int N = 9000, F = 7000;
int v[N + 1], f[2 * F + 1];

int main(){
    int n;
    fin >> n;
    for(int i = 1; i <= n; i++) fin >> v[i], f[2 * v[i]]++;
    int ans = 0;
    for(int i = 1; i <= n; i++)
        for(int j = i + 1; j <= n; j++){
            int add = f[v[i] + v[j]];
            if(v[i] == v[j]) add -= 2;
            ans += max(add, 0);
        }
    fout << ans;
    return 0;
}