import java.util.Scanner;

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();
	for(int i=0;i<N;i++){
	    if(i!=K-1){
		System.out.print(S.charAt(i));
	    }else{
		char sk = S.charAt(i);
		if(sk=='A') System.out.print('a');
		else if(sk=='B') System.out.print('b');
		else System.out.print('c');
	    }
	}
    }
}

/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