#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
//namaega184
int main(){
	char a[100001],b[100001];scanf("%s%s",a,b); 
	int la=strlen(a),lb=strlen(b);
	if(la<lb){b:printf("%s\n",b);return 0;}
	else if(la>lb){a:printf("%s\n",a);return 0;}
	else{
		for(int i=0;i<la;i++){
			if(a[i]=='7'&&b[i]=='4')goto b;
			else if(a[i]=='4'&&b[i]=='7')goto a;
			else if(a[i]<b[i])goto b;
			else if(a[i]>b[i])goto a;
		}	
	}
	printf("-1\n");
	return 0;
}