Problems
School Olympiad
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.
\InputFile
Nonnegative integers $a, b, c$ and $d$, not greater than $100$.
\OutputFile
Print the number of participants at school programming contest.
Input example #1
1 2 3 4
Output example #1
8