At school programming contest the participants were asked to solve two problems. $a$ students totally coped with the given tasks, and $b$ students solve nothing. How many participants were in competition if there was accepted $c$ correct submitions for the first task, and $d$ submitions for the second task. \InputFile Nonnegative integers $a, b, c$ and $d$, not greater than $100$. \OutputFile Print the number of participants at school programming contest.