#include <fstream>
#include <algorithm>

using namespace std;

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

int rez=0,i,n,j,k,v[9001],ap[7001];

int main()
{
    f>>n;

    for (i=1; i<=n; ++i) f>>v[i];

    for (i=1; i<=n; ++i)
    for (j=1; j<=i-1; ++j)
    {
    if ((v[i]+v[j])%2==0){
    ap[(v[i]+v[j])/2]++;
    if ((v[i]+v[j])/2==v[i]) rez-=2;
    }
    }

    for (i=1; i<=n; ++i) rez+=ap[v[i]];
    g<<rez<<'\n';
    return 0;
}