#include <iostream>
#include <fstream>
#pragma GCC optimize("Ofast")

using namespace std;

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

const int N = 9000, VAL_MAX = 7000;
int v[N + 1], f[VAL_MAX + 1];

void reset_f(int n){
    for(int i = 1; i <= VAL_MAX; i++) f[i] = 0;
}

int main(){
    int n, ans = 0;
    fin >> n;
    for(int i = 1; i <= n; i++) fin >> v[i];
    for(int i = n; i >= 1; i--){
        reset_f(n);
        for(int j = n; j >= 1; j--){
            if(j != i){
                if(2 * v[i] - v[j] <= VAL_MAX && 2 * v[i] - v[j] > 0) ans += f[2 * v[i] - v[j]];
                f[v[j]]++;
            }
        }
    }
    fout << ans;
    return 0;
}