School Olympiad

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.

Input

Nonnegative integers $a,b,c$ and $d$, not greater than $100$.

Output

Print the number of participants at school programming contest.

Examples