Problem Description

Given a list of words sorted according to an alien language, determine the order of characters in that alien alphabet. The words are given in lexicographically sorted order according to the alien language rules.

Input Format

First line contains n (number of words). Next n lines contain words in the alien language (lowercase letters only).

Output Format

A string representing the order of characters in the alien alphabet, or "INVALID" if no valid order exists.

Constraints

• 1 ≤ n ≤ 100

• 1 ≤ word length ≤ 100

• Words contain only lowercase English letters

• Words are given in sorted order according to alien language

• Return lexicographically smallest valid order if multiple exist

Sample Input/Output

4
wrt
wrf
er
ett

wertf

Explanation

From "wrt" and "wrf": t comes before f. From "wrt" and "er": w comes before e. From "er" and "ett": r comes before t. Building the order: w → e → r → t → f.