#include <bits/stdc++.h>
#pragma GCC optimize ("Ofast")

using namespace std;

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

const int MAX_N = 9006;
int n, x, val[MAX_N], frq[MAX_N];
long long sol;

int main (){
    ios_base::sync_with_stdio(false);
    fin.tie(nullptr);
    fout.tie(nullptr);

    fin>>n;
    for(int i=1; i<=n; i++){
        fin>>val[i];
        frq[ val[i] ]++;
    }

    for(int i=1; i < n; i++)
        for(int j=i+1; j<=n; j++){
            if(val[i]%2 != val[j]%2)
                continue;
            x = (val[i] + val[j]) / 2;
            sol += frq[x] - (val[i] == x) - (val[j] == x);
        }

    fout<<sol;
    return 0;
}