#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#define p(s) cout<<(s)<<endl
#define REP(i,n,N) for(int i=n;i<N;i++)
#define RREP(i,n,N) for(int i=N-1;i>=n;i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long ll;
using namespace std;

string A, B;
string ans;
int main(){
    cin>>A>>B;
    ans = A;
    if(A.size()<B.size()) ans=B;
    else if(A.size()==B.size()){
        REP(i, 0, A.size()){
            if((A[i]=='7'&&B[i]=='4')||((!(A[i]=='4'&&B[i]=='7'))&&A[i]<B[i])){
                ans=B;
                break;
            }else if((A[i]=='4'&&B[i]=='7')||(A[i]>B[i])){
                break;
            }
        }
    }
    p(ans);
    return 0;
}