#include <algorithm>
#define DIM 9005
#define MAX 7005
using namespace std;
int a[DIM],v[MAX],ap[MAX];
int n,nrt;
void read ()
{
    int i;
    scanf ("%d",&n);
    for (i=1; i<=n; ++i)
        scanf ("%d",&a[i]);
}
void solve ()
{
    int i,j;
    for (i=1; i<=n; ++i)
    {
        ++ap[a[i]];
        for (j=i+1; j<=n; ++j)
            if (!((a[i]+a[j])&1))
                ++v[(a[i]+a[j]>>1)];
    }
    for(i=1; i<=n; ++i)
		if (v[a[i]])
			nrt+=(v[a[i]]-ap[a[i]]+1);
    printf ("%d",nrt);
}
int main ()
{
    freopen ("medie.in","r",stdin);
    freopen ("medie.out","w",stdout);
    read ();
    sort (a+1,a+n+1);
    solve ();
    return 0;
}