#include <bits/stdc++.h>
using namespace std;

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

int n, v[9001], cnt, f[14001], f2[9001];

int main(){

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

    for(int i = 1; i < n; i++){
        for(int j = i + 1; j <= n; j++){
            f[v[i] + v[j]]++;
            if(v[i] + v[j] == 2 * v[i])
                f2[i]++;
            if(v[i] + v[j] == 2 * v[j])
                f2[j]++;
        }
    }

    for(int i = 1; i <= n; i++){
        cnt = cnt + f[v[i] * 2] - f2[i];
    }

    fout << cnt;
}