#include <bits/stdc++.h>
using namespace std;

// 基本テンプレート
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
#define P(x) cout << (x) << endl
#define p(x) cout << (x)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define MAT(type, c, m, n) vector<vector<type>> c(m, vector<type>(n));for(auto& r:c)for(auto& i:r)cin>>i;
#define VV(type, c, m, n, i) vector<vector<type>> c(m, vector<type>(n, i));
#define REP(i,a,n) for(int i=(a), i##_len=(n); i<i##_len; ++i)
#define RREP(i,a,n) for(int i=(a); i>n; --i)
#define perm(c) sort(ALL(c));for(bool c##p=1;c##p;c##p=next_permutation(ALL(c)))
#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter
#define MP make_pair
#define cauto const auto&
#define UNIQUE(v) v.erase( unique(ALL(v)), v.end() );
#define len(x) ((int)(x).size())
#define GET_ARRAY_SIZE(a)   (sizeof(a)/sizeof(a[0]))


// 型
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<string, string> pss;
typedef pair<string, int> psi;
typedef pair<int, string> pis;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long> vl;
typedef vector<long long> vll;
typedef vector<string> vs;

// 4近傍
//int dy[]={0, 1, 0, -1};
//int dx[]={1, 0, -1, 0};

// 8近傍
//int dy[]={0,0,1,-1,1,1,-1,-1};
//int dx[]={1,-1,0,0,1,-1,1,-1};
constexpr int dx[]={-1, 0, 1, -1, 1, -1, 0, 1};
constexpr int dy[]={1, 1, 1, 0, 0, -1, -1, -1};

// ull mod = (int)1e9 + 7;

// 定数
#define PI acos(-1.0)
constexpr int EPS = (1e-9);
constexpr int INF = 100000000;


int main() {
//	cout << setprecision(2) << fixed;

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

	string ans = "";
	if (len(a) != len(b))
		ans = len(a) > len(b) ? a : b;
	else
		REP(i, 0, len(a)) {
			if (a[i] == b[i]) continue;
			if (a[i] == '4' && b[i] == '7') { ans = a; break; }
			else if (a[i] == '7' && b[i] == '4') { ans = b; break; }
			ans = stoi(a) > stoi(b) ? a : b; break;
		}

	P(ans);




	return 0;
}