import Control.Applicative
  
main = do
    [_,n] <- map read.words <$> getLine
    str <- getLine
    putStrLn $ answer n str

answer :: Int -> String -> String
answer n str = take (n-1) str ++[upc (str !! (n-1))] ++ drop n str

upc 'A' = 'a'
upc 'B' = 'b'
upc 'C' = 'c'