#include <bits/stdc++.h>

using namespace std;

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

const int VMAX = 2e4;
int N, x, freq[VMAX], mx;
long long ans;

void max_self(int &a, int b) {
    a = max(a, b);
}

int main() {
    fin >> N;
    for(int i = 0; i < N; ++i) {
        fin >> x;
        ++freq[x];
        max_self(mx, x);
    }
    for(int i = 1; i <= mx; ++i) {
        ans += 1LL * (freq[i] - 2) * (freq[i] - 1) * freq[i] / 2LL;
        for(int j = 1; j < i; ++j)
            ans += 1LL * freq[i - j] * freq[i] * freq[i + j];
    }
    fout << ans << '\n';
}