#include <iostream>
#include <string>
using namespace std;
int main()
{
    string a, b; cin>>a>>b;

    if (a.size()>b.size()) {
        cout<<a<<endl;
    }
    else if (a.size()<b.size()) {
        cout<<b<<endl;
    }
    else {
        auto s=a, t=b;
        for(int i=0;i<a.size();++i) {
            if (a[i]=='4' and b[i]=='7')
                a[i]='7', b[i]='4';
            else if (a[i]=='7' and b[i]=='4')
                a[i]='4', b[i]='7';
        }
        cout<<(a>b ? s : t)<<endl;
    }
}