#!/usr/bin/env python

def read_input():
    with open('input.txt', 'r') as f:
        for line in f:
            yield line.strip()

def compare(a, b):
    if len(a) != len(b):
        return None

    change = None
    for i in range(len(a)):
        if a[i] != b[i]:
            if change is None:
                change = i
            else:
                return None
    return a[:change]+a[change+1:]

boxes = []
result = None
for box in read_input():
    for b in boxes:
        result = compare(box, b)
        if result:
            break
    if result:
        break
    boxes.append(box)

print(result)