#include <fstream>
#include <iostream>

#define MAXN 9005
#define NR 7005

using namespace std;

ifstream f ("medie.in");
ofstream g ("medie.out");

int n;
long long sol;
int v[MAXN], s[NR];

int main(){
    f >> n;

    for(int i = 1; i <= n; ++i){
        f >> v[i];
        s[v[i]]++;
    }
    for(int i = 1; i < n; ++i){
        for(int j = i + 1; j <= n; ++j){
            int k = 2 * v[j] - v[i];
            if(s[k]){
                sol += s[k];
            }
        }
    }
    g << sol;

}