import java.util.Scanner;

public class No293 {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		String[] numList = scanner.nextLine().split(" ");
		System.out.println(compare(numList[0], numList[1]) > 0 ? numList[0] : numList[1]);
		scanner.close();
	}
		
	private static int compare(String str1, String str2) {
		if(str1.length() > str2.length()) {
			return 1;
		}
		else if(str1.length() < str2.length()) {
			return -1;
		}
		else {
			for(int index = 0; index < str1.length(); index++) {
				char str1Char = str1.charAt(index);
				char str2Char = str2.charAt(index);
				if(str1Char == '4' && str2Char == '7' ) {
					return 1;
				}
				else if(str1Char == '7' && str2Char == '4') {
					return -1;
				}
				else {
					if(str1Char != str2Char) {
						return str1Char - str2Char;
					}
				}
			}
			return 0;
		}
	}
}