#include <stdio.h>

int main() {
	int a, b;
	int divisor;
	int result;

	scanf("%d%d", &a, &b);
	result = a;
	for (divisor = 1000000000; divisor > 0; divisor /= 10) {
		int digitA = a / divisor % 10;
		int digitB = b / divisor % 10;
		int sign;
		if ((digitA == 4 || digitA == 7) &&
			(digitB == 4 || digitB == 7)) {
			sign = -1;
		} else {
			sign = 1;
		}
		if (digitA * sign > digitB * sign) {
			result = a;
			break;
		} else if (digitA * sign < digitB * sign) {
			result = b;
			break;
		}
	}
	printf("%d\n", result);
	return 0;
}