Christmas Trees

Chef opened a tree-selling business this Christmas. He bought $N$ trees for $A$ rupees, and then sold $M$ trees for $B$ rupees. What was his profit this Christmas? If Chef instead made a loss of $X$ rupees, print $-X$. ### Input - The first and only line of input contains $4$ integers - $N, A, M$ and $B$. ### Output Output Chef's profit this christmas. ### Constraints - $1 \le M \le N \le 10