#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]&1) != (val[j]&1))
                continue;
            x = ((val[i]+val[j])>>1);
            sol += frq[x] - ((val[i] == x)<<1);
        }
    fout<<sol;
    return 0;
}