Algebra

The function f has the following properties:
* f(a,b) is defined for all positive integers a and b

* f(a,1) = a

* f(a,b) = 1 if b > a

* f(a + 1,b) = b[f(a,b) - f(a,b - 1)]

Compute f(4,1) + f(4,2) + f(4,3) + f(4,4).