#include<fstream>
using namespace std;
ifstream in("medie.in");
ofstream out("medie.out");
const int DMAX = 9005;

int v[DMAX],f[DMAX],n;

int main()
{

    in>>n;
    for(int i = 1 ; i <= n ; i++){
        in>>v[i];
        ++f[v[i]];
    }
    int medie,sol = 0;
    for(int i = 1 ; i < n ; i++)
    for(int j = i+1 ; j <= n ; j++){
        if((v[i]+v[j])%2 == 1)
            continue;
        medie = (v[i]+v[j])/2;
        if(medie == v[i])
            sol--;
        if(medie == v[j])
            sol--;
        sol+=f[medie];
    }
    out<<sol;
    return 0;
}