int s, t;

 if ( i <= 0 || j <= 0 )
    return(-1);
 else if ( i + j < k )
    return (i+j);
 else
 {
    s = 0;
    for (t = 1; t < k; t++)
    {
       s = s + F(i-t, j-t, k-1) + 1;               
    }
    return(s);
 }