#include <iostream>
#include <string>
#include <algorithm>

int comp(char a,char b){
    if(a=='4'&&b=='7')return 1;
    else
    if(a=='7'&&b=='4')return 0;
    return a>b;
}
using namespace std;
int main() {
    string A,B;
    cin>>A>>B;
    int n=max(A.size(),B.size());
    if(A.size()<n){
        cout<<B<<endl;
        return 0;
    }
    if(B.size()<n){
        cout<<A<<endl;
        return 0;
    }
    for(int i = 0; i < n; i++){
        if(comp(A[i],B[i])){
            cout<<A<<endl;
            return 0;
        }
        if(comp(B[i],A[i])){
            cout<<B<<endl;
            return 0;
        }
    }
    return 0;
}