#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <string>

#define repd(i,a,b) for (int i=(a);i<(b);i++)
#define rep(i,n) repd(i,0,n)
#define var auto
#define mod 1000000007
#define inf 2147483647
typedef long long ll;

using namespace std;

int inputValue(){
    int a;
    cin >> a;
    return a;
}

template <typename T>
void output(T a, int precision) {
    if(precision > 0){
        cout << fixed << setprecision(precision)  << a << "\n";
    }
    else{
        cout << a << "\n";
    }
}

// end of template

int main() {
    // source code
    string a, b;
    cin >> a >> b;
    
    string ret = "";
    
    if (a.size() != b.size()) {
        ret = (a.size() > b.size()) ? a : b;
    }
    else{
        rep(i, a.size()){
            if (a[i] == '4' && b[i] == '7'){
                ret = a;
                break;
            }
            else if (a[i] == '7' && b[i] == '4') {
                ret = b;
                break;
            }
        }
        if (ret == "") {
            stringstream ss;
            int A, B;
            ss << a;
            ss >> A;
            ss.clear();
            ss.str("");
            ss << b;
            ss >> B;
            output((A > B) ? A : B, 0);
            return 0;
        }
    }
    
    output(ret, 0);
    
    return 0;
}