#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#include <atcoder/dsu>
#include <atcoder/segtree>
using namespace atcoder;
using namespace std;
typedef long long ll;
#define rep(i,a,b) for(int i=a;i<b;i++)
using vi = vector<int>; 
using vvi = vector<vi>; 
using li =vector<ll>;
using lli=vector<li>;
using si =vector<char>;
using ssi =vector<si>;
const long long INF = 1e18;
const long long MOD=1e9+7;
//for (auto [key, val] : mp)
int main() { 
	string a,b;cin>>a>>b;
	int n=a.size();
	int m=b.size();
	if(n<m){cout<<b;return 0;}
	else if(n>m){cout<<a;return 0;}
	rep(i,0,min(n,m)){
		if(a[i]=='4'&&b[i]=='7'){cout<<a;return 0;}
		else if(a[i]=='7'&&b[i]=='4'){cout<<b;return 0;}
		else if(a[i]<b[i]){cout<<b;return 0;}
		else if(b[i]<a[i]){cout<<a;return 0;}
	}
	
}