{-# LANGUAGE BangPatterns, FlexibleContexts #-}

import Control.Applicative
import Control.Monad
import Data.List
import Debug.Trace (traceShow, traceShowM)
import qualified Data.ByteString.Char8 as B
import Data.Maybe

import Control.Monad.ST
import Data.Array.ST

inf :: Int
inf = 10 ^ 9 + 7

solve n m ss ts = runST $ do
    dp <- newArray ((0, 0), (n, m)) inf :: ST s (STUArray s (Int, Int) Int)
    writeArray dp (0, 0) 0
    forM_ [0 .. n] $ \i -> do
        forM_ [0 .. m] $ \j -> do
            v1 <- if all (>= 1) [i, j] then Just <$> readArray dp (i - 1, j - 1) else return Nothing
            v2 <- if i >= 1 then Just <$> readArray dp (i - 1, j) else return Nothing
            v3 <- if j >= 1 then Just <$> readArray dp (i, j - 1) else return Nothing
            let vs = catMaybes [v1, v2, v3]
            when (not $ null vs) $ do
                if isJust v1
                    then do
                        if (ss `B.index` (i - 1)) == (ts `B.index` (j - 1))
                            then writeArray dp (i, j) (minimum $ [fromJust v1] ++ fmap (+ 1) (catMaybes [v2, v3]))
                            else writeArray dp (i, j) (minimum vs + 1)
                    else writeArray dp (i, j) (minimum vs + 1)
    readArray dp (n, m)

main :: IO ()
main = do
    [n, m] <- fmap read . words <$> getLine :: IO [Int]
    ss <- B.getLine
    ts <- B.getLine
    print $ solve n m ss ts