#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

bool not47(char c){
	return (c != '4') && (c != '7');
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	string a, b;
	cin >> a >> b;

	int asz = a.size();
	int bsz = b.size();

	if(asz > bsz){
		cout << a << endl;
		exit(0);
	}

	if(asz < bsz){
		cout << b << endl;
		exit(0);
	}

	rep(i, asz){
		if(a[i] == b[i]) continue;
		if(not47(a[i]) || not47(b[i])){
			if(a[i] > b[i]){
				cout << a << endl;
			}else{
				cout << b << endl;
			}
			exit(0);
		}else{
			if(a[i] == '4'){
				cout << a << endl;
			}else{
				cout << b << endl;
			}
			exit(0);
		}
	}
}