#include <iostream>
#include <string>

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(0);
    
    std::string a, b;
    std::cin >> a >> b;
    std::string output;
    if(a.size()!=b.size()){
        output = (a.size()>b.size()) ? a : b;
    }else{
        for(unsigned int i=0; i<a.size(); ++i){
            if(a[i]!=b[i]){
                if(a[i]=='4' && b[i]=='7') output = a;
                else if(a[i]=='7' && b[i]=='4') output = b;
                else output = (a[i]>b[i]) ? a : b;
                break;
            }
        }
    }
    std::cout << output << "\n";
    return 0;
}