#include <fstream>
#include <algorithm>

using namespace std;

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

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

int main()
{
    f>>n;

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

    for (i=2; i<=n-1; ++i)
    {
        for (j=1; j<=i-1; ++j)
        for (k=i+1; k<=n; ++k)
        if (v[k]==2*v[i]-v[j]) rez++;
    }
    g<<rez<<'\n';
    return 0;
}