#include <bits/stdc++.h>
#define ll long long
#define MOD 9001
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
ll triplete, n, v[9005], fv[7005];

int main() {
    fin >> n;
    for (int i = 1; i <= n; ++i) {
        fin >> v[i];
        ++fv[v[i]];
    }
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= n; ++j)
            if (i != j && ((fv[2 * v[i] - v[j]] > 0 && v[i] != v[j]) || fv[2 * v[i] - v[j]] > 2))
                triplete += fv[2 * v[i] - v[j]] - 2;
    fout << abs(triplete) / 2;
    return 0;
}