#include <bits/stdc++.h>

#define MOD 1000000007
#define INF (1<<30)
#define INFL (1<<62)
#define pe(str) return cout<<(str)<<endl,0
#define px(str) cout<<(str)<<endl,exit(0)
#define accu(ac,v) cout<<setprecision(ac)<<v<<endl
#define bpc(a) __builtin_popcount(a)
#define pr(str) cout<<(str)<<endl
#define prpr(str) cout<<str<<endl
#define db(str) cerr<<"debug:"<<(str)<<endl
#define dbdb(str) cerr<<"debugs:"<<str<<endl
#define re(i,n) for(int i=0;i<(n);i++)
#define rre(i,n) for(int i=(n);i>=0;i--)
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b);i>=(a);i--)
#define bw(a,b,c) (((a)<=(b))&&((b)<=(c)))
#define hello cout<<"hello"<<endl
#define spre(a) setprecision(a)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define s3(C,T,F) ((C)?(T):(F))
#define enl cout<<endl
#define declare(t,n) t n;cin>>n

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;
typedef complex<int> point;

template<class InputIterator>
void dump(InputIterator first,InputIterator last,char delim=' '){
	for(InputIterator it=first;it!=last;it++){
		if(it!=first)cout<<delim;
		cout<<*it;
	}
}

ull p(ull x,ull y){
	if(y==0) return 1;
	ull A=p(x,y/2)%MOD;
	return (y&1?((A*A)%MOD*x)%MOD: (A*A)%MOD);
	// return (y&1?((A*A)*x):(A*A));
}

template<class T>
void swp(T &a,T &b){
	T t=a; a=b; b=t;
}

int main(void){
	string a,b;
	cin>>a>>b;
	if(a.size()>b.size())pe(a);
	else if(a.size()<b.size())pe(b);
	re(i,a.size()){
		if(a[i]!=b[i]){
			if(a[i]!='4'&&a[i]!='7') pe(a[i]>b[i]?a:b);
			else{
				if(a[i]=='4'&&b[i]=='7')pe(a);
				else if(a[i]=='7'&&b[i]=='4')pe(b);
				pe(a[i]>b[i]?a:b);
			}
		}
	}
	pe(a);
	return 0;
}