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

int main(void){
	char a[16], b[16];
	char *ans;
	int i=0;
	
	scanf("%s%s",a,b);
	
	if(strlen(a) > strlen(b)) ans = a;
	else if(strlen(a) < strlen(b)) ans = b;
	else{
		while(1){
			if(a[i] != b[i]){
				if(a[i] =='7' && b[i] == '4'){
					ans = b;
					break;
				}else if(a[i] =='4' && b[i] == '7'){
					ans = b;
					break;
				}else if(a[i] > b[i]){
					ans = a;
					break;
				}else if(a[i] < b[i]){
					ans = b;
					break;
				}
			}
			i++;
		}
	}
	printf("%s",ans);
	return 0;
}