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();
		StringBuilder ans =new StringBuilder(S);
		if(S.charAt(K-1)=='A'){		
			ans.setCharAt((K-1), 'a');
		}
		if(S.charAt(K-1)=='B'){		
			ans.setCharAt((K-1), 'b');
		}
		if(S.charAt(K-1)=='C'){		
			ans.setCharAt((K-1), 'c');
		}

		System.out.println(ans);	
		}
	}

/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