KöMaL 2025 C1872

Peti has written the integers from one to fifty on the blackboard. In each step, he chooses two numbers ($a$ and $b$) on the board, erases them and replaces them with the number defined by the formula $a^2b-6a^2-7ab+42a+6b-30.$ He repeats this until a single number remains on the board. What can this number be?