#include <stdio.h>
#include <string.h>

int main(int argc, char* argv[])
{
	char	c_maxNum[] = "2000000000";
	char	c_num1[sizeof("2000000000")] = "";
	char	c_num2[sizeof("2000000000")] = "";
	scanf("%s%s", c_num1, c_num2);

	if (strlen(c_num1) < strlen(c_num2)) {
		printf("%s\n", c_num2);
	}
	else if (strlen(c_num2) < strlen(c_num1)) {
		printf("%s\n", c_num1);
	}
	else {
		for (size_t size = 0; size < strlen(c_maxNum); size++) {
			if (c_num1[size] == c_num2[size]) {
				continue;
			}

			if (c_num1[size] == '4' && c_num2[size] == '7') {
				printf("%s\n", c_num1);
			}
			else if (c_num1[size] == '7' && c_num2[size] == '4') {
				printf("%s\n", c_num2);
			}
			else if (c_num1[size] < c_num2[size]) {
				printf("%s\n", c_num2);
			}
			else {
				printf("%s\n", c_num1);
			}
			break;
		}
	}

	return 0;
}