#include <bits/stdc++.h>

using namespace std;

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

int n, v[9005], fr[7005];
long long ans;


int main()
{
    fin >> n;
    for (int i = 1; i <= n; ++i)
    {
        fin >> v[i];
        ++fr[v[i]];
    }
    for (int i = 1; i < n; ++i)
    {
        for (int j = i + 1; j <= n; ++j)
        {
            if (v[i] % 2 != v[j] % 2) continue;
            fr[v[i]]--;
            fr[v[j]]--;
            int x = (v[i] + v[j]) / 2;
            ans += fr[x];
            fr[v[i]]++;
            fr[v[j]]++;
        }
    }
    fout << ans;
    fin.close();
    fout.close();
    return 0;
}