Represnt short form of year in java

The problem is to represent any year in its short form. For example, if you want to find the short form of year 4 to the powern7. 4^7=16384. So the short form is 84.My code is given below. it does not work for the last condition if my base is 123456 and power is 123456 import java.util.Scanner;

public class Main {

public static void main(String[] args) {

    System.out.println(lastTwoDig(4,7));
    System.out.println(lastTwoDig(5,5));
    System.out.println(lastTwoDig(6,4));
    System.out.println(lastTwoDig(123456 ,123456));
}
public static String lastTwoDig(long x,long y){
     double year =  (double) Math.pow(x, y);
     String yearString = Double.toString(year);
     String lastTwo = yearString.substring( yearString.length()-4,yearString.length()-2);
     if(yearString.length()>2){
        return lastTwo;
     }
    return yearString;
}

}