import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int K = sc.nextInt();
        String S = sc.next();
        char s = S.charAt(K-1);
        String sn = String.valueOf(s);
        if(sn.equals("A")){
            sn = "a";
        }
        else if(sn.equals("B")){
            sn = "b";
        }
        else if(sn.equals("C")){
            sn = "c";
        }

        if(K==1){
            System.out.println(sn + S.substring(1));
        }
        else if(K==2){
            System.out.println(S.charAt(0) + sn + S.substring(2));
        }
        else if(K>=3){
            System.out.println(S.substring(0, K-1) + sn + S.substring(K));
        }
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz