// yukicoder: No.293 4>7の世界
// 2019.4.14 bal4u

#include <stdio.h>

#define gc() getchar()
#define pc(c) putchar(c)
int ins(char *s)  // 文字列の入力　スペース以下の文字で入力終了
{
	char *p = s;
	do *s = gc();
	while (*s++ > ' ');
	*--s = 0;
	return s - p;
}
void outs(char *s) { while (*s) pc(*s++); }

char A[15]; int wa;
char B[15]; int wb;

int cmp(int w1, char *s1, int w2, char *s2)
{
	if (wa != wb) return wa - wb;
	while (*s1) {
		if (*s1 == '4' && *s2 == '7') return 1;
		else if (*s1 == '7' && *s2 == '4') return -1;
		else if (*s1 != *s2) return *s1 - *s2;
		s1++, s2++;
	}
	return 0;
}

int main()
{
	wa = ins(A), wb = ins(B);
	outs(cmp(wa, A, wb, B) > 0 ? A : B);
	pc('\n');
	return 0;
}