#include <bits/stdc++.h>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int n, v[9005], fr[7005], mx, fr2[7005];
long long ans;
int main()
{
    fin >> n;
    for (int i = 1; i <= n; ++i)
    {
        fin >> v[i];
        mx = max(mx, v[i]);
        fr[v[i]]++;
    }
    for (int i = 1; i <= n; ++i)
    {
        for (int j = 1; j <= mx; ++j)
        {
            if (v[i] == j) continue;
            int x = 2 * j - v[i];
            if (x < 0 || x > mx) continue;
            ans += fr2[x] * fr[j];
        }
        ++fr2[v[i]];
    }
    for (int i = 1; i <= mx; ++i)
    {
        if (fr[i] < 3) continue;
        int x = fr[i] - 1;
        ans += (x * (x - 1)) / 2 * (x + 1);
    }
    fout << ans;
    fin.close();
    fout.close();
    return 0;
}