#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0; i<(n); i++)
typedef long long ll;

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