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

#define ll long long

using namespace std;

int main(){
    
    string a,b;
    cin>>a>>b;
    
    string ta,tb;
    ta=a,tb=b;

    for(int i=0;i<a.length();++i){
        if(a[i]=='4') a[i]='7';
        else if(a[i]=='7') a[i]='4';
    }
    for(int i=0;i<b.length();++i){
        if(b[i]=='4') b[i]='7';
        else if(b[i]=='7') b[i]='4';
    }

    int A=0,B=0;
    int digit=1;
    for(int i=a.length()-1;i>=0;--i){
        A+=digit*(a[i]-'0');
        digit*=10;
    }
    digit=1;
    for(int i=b.length()-1;i>=0;--i){
        B+=digit*(b[i]-'0');
        digit*=10;
    }

    if(A<B) cout<<tb<<endl;
    else cout<<ta<<endl;

    return 0;
}