#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <algorithm>
#include <map>
#include <set>
#include <cstdio>
#include <cmath>
#define rep(i,l,n) for(lint i=l;i<n;i++)
#define rer(i,l,n) for(lint i=l;i<=n;i++)
#define all(a) a.begin(),a.end()
#define o(a) cout<<a<<endl
#define pb(a) push_back(a)
#define mk(a,b) make_pair(a,b)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef vector<int> vi;
typedef vector<lint> vli;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

int main(){
	string a[2];
	cin>>a[0]>>a[1];
	if(a[0].size()>a[1].size()) o(a[0]);
	else if(a[0].size()<a[1].size()) o(a[1]);
	else{
		int ans=0;
		rep(i,0,a[0].size()){
			if(a[0][i]==a[1][i]) continue;
			else if(a[0][i]=='4' && a[1][i]=='7'){
				ans=0; break;
			}else if(a[0][i]=='7' && a[1][i]=='4'){
				ans=1; break;
			}else if(a[0][i]>a[1][i]){
				ans=0; break;
			}else{
				ans=1; break;
			}
		}
		if(ans) o(a[1]);
		else o(a[0]);
	}
}