Problems

# Dragon

Every S-leg have 1 head. Find the quantity of legs N in K-head dragon, if they have A head and B legs together.

#### Input

4 numbers: S, K, A, B. Every numbers is <= 1000.

#### Output

The quantity of legs of dragon. If input data is contradiction, then output in file -1, if you have some ways of solution – find free from it.

Time limit 1 second
Memory limit 64 MiB
Input example #1
4 7 35 36

Output example #1
2