package quki.algorithm.string;

import java.util.Scanner;

public class P1032 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int n = sc.nextInt();

sc.nextLine();

String ans = "";

for (int j = 0; j < n; j++) {

String line = sc.nextLine();

// String[] s = line.split("\\."); //조심!!

if (j == 0) { // 가장 초기 문자열 초기화

ans = line;

} else {

StringBuffer sb = new StringBuffer();

for (int i = 0; i < line.length(); i++) {

char orig = ans.charAt(i);

char curr = line.charAt(i);

if (orig != curr) {

orig = '?';

}

sb.append(orig);

}

ans = sb.toString();

}

}

System.out.println(ans);

}

}