Problem:
Score : 300 points

Problem Statement

You are given a string S of length N and another string T of length M.
These strings consist of lowercase English letters.

A string X is called a good string when the following conditions are all met:

Let L be the length of X. L is divisible by both N and M.

Concatenating the 1-st, (\frac{L}{N}+1)-th, (2 \times \frac{L}{N}+1)-th, ..., ((N-1)\times\frac{L}{N}+1)-th characters of X, without changing the order, results in S.

Concatenating the 1-st, (\frac{L}{M}+1)-th, (2 \times \frac{L}{M}+1)-th, ..., ((M-1)\times\frac{L}{M}+1)-th characters of X, without changing the order, results in T.

Determine if there exists a good string. If it exists, find the length of the shortest such string.

Constraints

1 \leq N,M \leq 10^5

S and T consist of lowercase English letters.

|S|=N

|T|=M

Input

Input is given from Standard Input in the following format:

N M
S
T

Output

If a good string does not exist, print -1; if it exists, print the length of the shortest such string.

Sample Input 1

3 2
acp
ae

Sample Output 1

6

For example, the string accept is a good string.
There is no good string shorter than this, so the answer is 6.

Sample Input 2

6 3
abcdef
abc

Sample Output 2

-1

Sample Input 3

15 9
dnsusrayukuaiia
dujrunuma

Sample Output 3

45

Solution in Haskell (GHC 7.10.3) (judge verdict: Wrong Answer)

import Data.Maybe

main :: IO ()
main = do
  [n,m] <- map read . words <$> getLine
  s <- getLine
  t <- getLine
  putStrLn $ fromMaybe "-1" $ solve n m s t

solve :: Int -> Int -> String -> String -> Maybe String
solve n m s t = if b then Just (build p str) else Nothing
  where
    l = lcm n m
    n' = l `div` n
    m' = l `div` m
    l' = lcm n' m'
    b = all (\(i,j) -> s !! i == t !! j) [(p`div`n',p`div`m') | x <- [0..(l`div`l'-1)], let p = x*l']
    (str,p) = if n > m then (s,n') else (t,m')

build :: Int -> String -> String
build p str = concat [s:(replicate (p-1) 'a') | s <- str]