#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream in ("medie.in");
ofstream out("medie.out");
int a[9100], frcv[9100], val[9100], n, maxi, rez;
int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
    {
        in>>a[i]; frcv[a[i]]++;
        maxi=max(maxi, a[i]);
    }
    sort(a+1, a+n+1);
    for(int i=1; i<=n-1; i++)
        for(int j=i+1; j<=n; j++)
            if((a[i]+a[j])%2==0)
                val[(a[i]+a[j])/2]++;
    for(int i=1; i<=n; i++)
        rez+=val[a[i]]-frcv[a[i]]+1;
    out<<rez;
    return 0;
}