#include<bits/stdc++.h>
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3f
#define EPS (1e-10)
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;
typedef long long ll;
typedef pair<int, int>P;

int main() {
	string a, b; cin >> a >> b;
	if (a.size() != b.size()) {
		if (a.size() > b.size())cout << a << endl;
		else cout << b << endl;
		return 0;
	}
	rep(i, a.size()) {
		if (a[i] != b[i]) {
			if (a[i] == '4'&&b[i] == '7')cout << a << endl; 
			else if (a[i] == '7'&&b[i] == '4')cout << b << endl;
			else if (a[i] > b[i])cout << a << endl;
			else cout << b << endl;
			return 0;
		}
	}
}