package Practice;

import java.io.BufferedReader;

import java.io.InputStreamReader;

public class B1032 { //¸í·É ÇÁ·ÒÇÁÆ®

public static void main(String[] args) throws Exception {

// TODO Auto-generated method stub

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

StringBuilder sb = new StringBuilder();

int N = Integer.parseInt(br.readLine());

String[] files = new String[N];

for(int i = 0; i < N; i++) files[i] = br.readLine();

for(int i = 0; i < files[0].length(); i++) {

char c = files[0].charAt(i);

for(int j = 1; j < N; j++) {

if(c != files[j].charAt(i)) {

c = '?';

break;

}

}

sb.append(c);

}

System.out.println(sb);

}

}