#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];
    for (int p = 1; p <= n; ++p)
        for (int i = 1; i <= n; ++i)
            for (int j = 1; i != p && j < i; ++j)
                if (j != p && v[p] * 2 == v[i] + v[j])
                    ++triplete;
    fout << triplete;
    return 0;
}