Social networks

On Instagram, there are $A$ students registered, while on Facebook, there are $B$ students. Among these, $C$ students are registered on both social networks. Additionally, $D$ students do not use the Internet or social networks at all. Calculate the total number of students in the class.

Input

The input consists of a single line containing four integers: $A$, $B$, $C$, and $D$.

Output

Output a single number representing the total number of students in the class.

Examples