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

char *comp(char *a, char *b, int alen, int blen) {
	int i;
	if (alen != blen) return alen > blen ? a : b;
	else {
		for (i = 0; i < alen; i++) {
			if (a[i] == b[i]) continue;
			else if (a[i] == '4' && b[i] == '7') return a;
			else if (a[i] == '7' && b[i] == '4') return b;
			else return a[i] > b[i] ? a : b;
		}
	}
}

int main() {
	char a[11], b[11];

	scanf("%s %s", a, b);
	printf("%s\n", comp(a, b, strlen(a), strlen(b)));
	
	return 0;
}