#include <bits/stdc++.h>
#define ll long long
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
ll n, rez, v[9005], ap[7005];

int main() {
    fin >> n;
    for (int i = 1; i <= n; ++i) {
        fin >> v[i];
        ++ap[v[i]];
    }
    for (int i = 1; i <= n; ++i) {
        for (int j = i + 1; j <= n; ++j) {
            if ((v[i] + v[j]) % 2 != 0)
                continue;
            if (ap[(v[i] + v[j]) / 2] >= 1 && v[j] != v[i])
                rez += ap[(v[i] + v[j]) / 2];
            else if (ap[(v[i] + v[j]) / 2] >= 3)
                rez += ap[(v[i] + v[j]) / 2] - 2;
        }
    }
    fout << rez;
    return 0;
}