Problems

# Dragon

# Dragon

Every centipede has $S$ legs and $1$ head. Find the quantity of legs $N$ of a $K$-head dragon, if there is total of $A$ heads and $B$ legs.

#### Input

4 numbers: $S$, $K$, $A$, $B$. Every numbers is less or equals to 1000. It is guaranteed that there is at least one dragon and one centipede.

#### Output

The number of legs of a dragon. If input data is contradiction, then output $-1$, if there is more than one answer, output any one of them.

Input example #1

4 7 35 36

Output example #1

2