International Mathematical Olympiad 1981 Problem 6

The function $f(x, y)$ satisfies

(1) $f(0, y) = y + 1$,

(2) $f(x + 1, 0) = f(x, 1)$,

(3) $f(x + 1, y + 1) = f(x, f(x + 1, y))$,

for all non-negative integers $x, y$. Determine $f(4, 1981)$.