import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Word_fast {
    public static void main(String[] args) throws Exception {
        BufferedReader b = new BufferedReader( new InputStreamReader(System.in));
        String s = b.readLine();
        int n = Integer.parseInt(s);
        String str[] = new String[n];
        String stt[] = new String[n];
        for (int i =0; i<=n-1;i++){            
            str[i] = b.readLine();
            stt[i]=str[i];
        }
        for (int i =1; i<=n-1;i++){
             
            if(str[i-1].length() <= str[i].length()){
                s=str[i-1];
                str[i]=str[i-1];
                str[i-1]=s;
            }
        }
        String shortest = str[n-1];
        for(int i=shortest.length()-1;i>=0;i--){
            for(int j=i; j<=shortest.length()-1;j++){
                String st=shortest.substring(j-i,j+1);          
                int count=0;
                for(int k=0; k<=str.length-1;k++){
                    if(stt[k].contains(st)){
                        count++;
                    }
                }
                if(count==n){
                    System.out.println(st);
                    System.exit(0);
                }
            }
        }
    }
}