{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BinaryLiterals #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE NumDecimals #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.ST

import Data.Char
import Data.List
import Data.Maybe

import qualified Data.ByteString.Char8 as B

import Data.Array.ST.Safe
import Data.STRef

import Debug.Trace
import Text.Printf

readInt = readLn :: IO Int
readInts = map ( fst . fromJust . B.readInt ) . B.words <$> B.getLine
readNInts = readInt >>= flip replicateM readInt

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

modifyArray a i f = writeArray a i =<< f <$> readArray a i

main = do
	[ n, m ] :: [Integer] <- map fromIntegral <$> readInts
	s <- getLine
	t <- getLine
	let
		l = lcm n m
		ps = zip s $ map ( * ( l `div` n ) ) [ 0 .. ]
		qs = zip t $ map ( * ( l `div` m ) ) [ 0 .. ]
	print $ if f ps qs
		then l
		else -1

f [] _ = True
f _ [] = True
f list1@( ( c1, p1 ) : rest1 ) list2@( ( c2, p2 ) : rest2 )
	| p1 < p2 = f rest1 list2
	| p1 > p2 = f list1 rest1
	| c1 == c2 = f rest1 rest2
	| otherwise = False