import java.util.*; 

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();
     

  		String[] s1=s.split("");
		
    
 	if(s1[k-1].equals("A")){
    		s1[k-1]="a";
    	}else if(s1[k-1].equals("B")){
     		s1[k-1]="b";
   	 }else{
    	s1[k-1]="c";
    }
  		String ans="";
		for(String a:s1){
    		ans+=a;
        }
  		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