#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
using namespace std;
#define Ma_PI 3.141592653589793
#define eps 0.00000000000000000000000001
#define LONG_INF 10000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define REP(i,n) for(long long i = 0;i < n;++i)
#define seg_size 262144
int main() {
	string a, b;
	cin >> a >> b;
	if (a.length() > b.length()) {
		cout << a << endl;
	}
	else if (a.length() < b.length()) {
		cout << b << endl;
	}
	else {
		for (int i = 0;i < a.length();++i) {
			if ((a[i] == '4' || a[i] == '7') && (b[i] == '4' || b[i] == '7')) {
				if (a[i] == '4')a[i] = '7';
				else a[i] = '4';
				if (b[i] == '4') b[i] = '7';
				else b[i] = '4';
			}
			if(a[i] > b[i]){
				if ((a[i] == '4' || a[i] == '7') && (b[i] == '4' || b[i] == '7')) {
					if (a[i] == '4')a[i] = '7';
					else a[i] = '4';
					if (b[i] == '4') b[i] = '7';
					else b[i] = '4';
				}
				cout << a << endl;
				return 0;
			}
			else if (a[i] < b[i]) {
				if ((a[i] == '4' || a[i] == '7') && (b[i] == '4' || b[i] == '7')) {
					if (a[i] == '4')a[i] = '7';
					else a[i] = '4';
					if (b[i] == '4') b[i] = '7';
					else b[i] = '4';
				}
				cout << b << endl;
				return 0;
			}
			if ((a[i] == '4' || a[i] == '7') && (b[i] == '4' || b[i] == '7')) {
				if (a[i] == '4')a[i] = '7';
				else a[i] = '4';
				if (b[i] == '4') b[i] = '7';
				else b[i] = '4';
			}
		}
		cout << a << endl;
	}
}