module Main where

import Data.List
import Control.Monad
import Debug.Trace

change :: Char -> Char
change 'A' = 'a'
change 'B' = 'b'
change 'C' = 'c'

solve :: Int -> String -> String
solve k s = map (\(w,i) -> if i == k then change w else w) $ zip s [1..]

main = do
    nk <- getLine
    let [n, k] = read <$> words nk :: [Int]
    s <- getLine
    putStrLn $ solve k s
