#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");

int n, m, i, j, x, y, z, v[9010], f[9010], sol[9010], s;

int main(){
    fin>>n;
    for(i=1; i<=n; i++){
        fin>>v[i];
        f[ v[i] ]++;
    }
    for(i=1; i<n; i++){
        for(j=i+1; j<=n; j++){
            if( (v[i]+v[j])%2==0 ){
                x=(v[i]+v[j])/2;
                sol[x]++;
            }
        }
    }
    for(i=1; i<=n; i++){
        x=v[i];
        s+=sol[x]-f[x]+1;
    }
    fout<<s;
}