import java.util.*;
class Main {
public static void main(String[] args) {
System.out.println(new Main().addUnderscoreToLetters("abcDrgeifkHfhfgh"));
}
public String addUnderscoreToLetters(String words){
char[] charArray = words.toCharArray();
List<Character> charList = new LinkedList<>();
for(char character: charArray){
charList.add(character);
}
for(int i = 0 ; i < charList.size()-1; i++){
if((int)charList.get(i) >= (int)'a' && (int)charList.get(i) <= (int)'z' && (int)charList.get(i+1) >= (int)'A' && (int)charList.get(i+1) <= (int)'Z'){
charList.add(i+1,'_');
}
}
return charList.toString();
}
}
// Input String s = "abcDrgeifkHfhfgh"; Add '_' wherever ther is a small letter followed by a capital letter
