There are two sets of real numbers: $S_1 = [0, N_1]$, $S_2 = [0, N_2]$, where $N_1$ and $N_2$ are positive integers. From $S_1$ a value $x_1$ and from $S_2$ a value $x_2$ is chosen.
What will be the probability that $x_1 + x_2 < y$, where $y$ is an integer number?