#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  for(int i=0;i<(n);++i)
#define FORq(i, m, n) for(int i = (m);i <= (n);++i)
#define SCD(n) scanf("%d",&n)
#define SCD2(m,n) scanf("%d%d",&m,&n)
#define SCD3(m,n,k) scanf("%d%d%d",&m,&n,&k)
#define PB push_back
#define MP make_pair
#define ARSCD(A,N) REP(i,N){SCD(A[i]);}
#define ARSCD1(A,N) FORq(i,1,N){SCD(A[i]);}
#define PRINTD(n) printf("%d\n",n)
#define PRINTLLD(n) printf("%lld\n",n)
#define DEBUG printf("%s\n","debug")
#define fst first
#define snd second
#define IN(x,S) (S.count(x) != 0)
using namespace std;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef long long ll;
//////////////////////////////////////////////////////

int main(){
	string A,B;
	
	cin >> A >> B;
	int Al = A.length();
	int Bl = B.length();
	
	if (Al < Bl){
		cout << B << "\n";	
		return 0;
	}
	
	if (Al > Bl){
		cout << A << "\n";
		return 0;
	}
	
	REP(i,Al){
		if (A[i] == B[i]){
			continue;
		}
		
		if ((A[i] == '4') and (B[i] == '7')){
			cout << A << "\n";
			return 0;
		}
		
		if ((A[i] == '7') and (B[i] == '4')){
			cout << B << "\n";
			return 0;
		}
		
		if (A[i] < B[i]){
			cout << B << "\n";
			return 0;
		}else{
			cout << A << "\n";
			return 0;
		}
		
	}
	
	return 0;
}