import java.util.*;

public class Main{
	public  static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int k = scan.nextInt();
      	String s = scan.next();
      	System.out.print(s.substring(0, k-1));
      	if(s.substring(k-1, k).contains("A")){
        	System.out.print("a");
        }
      	else if(s.substring(k-1, k).contains("B")){
        	System.out.print("b");
        }
        else if(s.substring(k-1, k).contains("C")){
        	System.out.print("c");
        }
      	System.out.print(s.substring(k, n));
    }
}
/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