#include<iostream>
#include<string>
using namespace std;

string iQueryMaxCommString(string s1, string s2);
int main(){
    string strA, strB;
    cin>>strA>>strB;
    cout << iQueryMaxCommString(strA, strB) <<endl;

    return 0;
}

string iQueryMaxCommString(string s1, string s2){
    int flag = 1;
    string maxlen;
    string null = "null";
    int len1 = s1.length();
    int len2 = s2.length();
    if(len1 == 1){
        if(s2.find(s1) != s2.npos)
            return s1;
        else
            return null;
    }
    if(len2 == 1){
        if(s1.find(s2) != s1.npos) return s2;
        else
            return null;
    }

    if(len1 > len2){
            //cout<<"----------len1 > len2----------"<<endl;
        for(int i = 0; i < len2 - 1; i++){
            for(int j = 1; j <= len2; j++){
                string temp = s2.substr(i, j - i);
                if(s1.find(temp) != s1.npos){
                    flag = 0;
                    if(temp.length() > maxlen.length()) maxlen = temp;
                }
            }
        }
    //cout<<"flag = "<<flag<<endl;
    //cout<<"maxlen = "<<maxlen<<endl;
        if(flag == 1)
            return null;
        else
            return maxlen;
    }
    else{
        //cout<<"----------len1 <= len2----------"<<endl;
        for(int i = 0; i < len1 - 1; i++){
            for(int j = 1; j <= len1; j++){
                string temp = s1.substr(i, j - i);
                if(s2.find(temp) != s2.npos){
                    flag = 0;
                    if(temp.length() > maxlen.length()) maxlen = temp;
                }
            }
        }
       //cout<<"flag = "<<flag<<endl;
       //cout<<"maxlen = "<<maxlen<<endl;
        if(flag == 1)
            return null;
        else
            return maxlen;

    }

}