#pragma region Macros

#include <bits/extc++.h>
using namespace std;
using namespace __gnu_pbds;
// using namespace __gnu_cxx;

#define TO_STRING(var) # var
#define pb emplace_back
#define int ll
#define endl '\n'

using ll = long long;
using ld = long double;
const ld PI = acos(-1);
const ld EPS = 1e-10;
const int INF = 1 << 30;
const ll INFL = 1LL << 62;
// const int MOD = 998244353;
const int MOD = 1000000007;

int ceil(int x, int y) {return (x > 0 ? (x + y - 1) / y : x / y);}
int POW(int x, int y) {
    if (y != (int)(y) or y < 0 or x != 0 && x != 1 && y > 64) {cout << "Error" << endl;return 0;}
    if (y == 0) return 1;
    if (y % 2 == 0) return POW(x * x, y / 2);
    return x * POW(x, y - 1);
}

__attribute__((constructor))
void constructor() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);
}

#pragma endregion

bool com(char a, char b) {
	if (a == '4' && b == '7') return true;
    else if (a == '7' && b == '4') return false;
    return a > b;
}

signed main() {
    string A, B;
	cin >> A >> B;
	if (A.size() > B.size()) cout << A << endl;
	else if (A.size() < B.size()) cout << B << endl;
	else {
		bool flag = true;
		for (int i = 0; i < (int)A.size(); i++) {
			if (A[i] != B[i] && com(A[i], B[i])) {
				flag = true;
				break;
			}
			if (A[i] != B[i] && !com(A[i], B[i])) {
				flag = false;
				break;
			}
		}
		cout << (flag ? A : B) << endl;
	}
}