As a hint, if you unroll the recurrence, you'll see that it evaluates to
U(n) = (n - 1) + (n - 2) + ... + 3
Also, it might help to know that if you evaluate n(n + 1) / 2 (the sum of the first n positive natural numbers), you get back the sequence 0, 1, 3, 6, 10, 15, 21, etc. You can formalize the result by using a proof by induction.
Hope this helps!