# Dragon

Easy

Execution time limit is 1 second

Runtime memory usage limit is 64 megabytes

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.

## Examples

Input #1

Answer #1

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