OMM 1993 Problem 4

$f(n,k)$ is defined by (1) $f(n,0) = f(n,n) = 1$ and (2) $f(n,k) = f(n-1,k-1) + f(n-1,k)$ for $0 < k < n$ . How many times do we need to use (2) to find $f(3991,1993)$ ?