using System;

public class Solution{

public string solution(string phone_number){

string am = phone_number;

am = am.Remove(0, phone_number.Length - 4);

for (int a=0;a < phone_number.Length-3;){

am = am.Insert(0, "*");

a++;}

string answer = am; 

return answer;}}

빨리 기초 때고 싶다