human
Solve in Python:
Fox Ciel is going to publish a paper on FOCS (Foxes Operated Computer Systems, pronounce: "Fox"). She heard a rumor: the authors list on the paper is always sorted in the lexicographical order. 

After checking some examples, she found out that sometimes it wasn't true. On some papers authors' names weren't sorted in lexicographical order in normal sense. But it was always true that after some modification of the order of letters in alphabet, the order of authors becomes lexicographical!

She wants to know, if there exists an order of letters in Latin alphabet such that the names on the paper she is submitting are following in the lexicographical order. If so, you should find out any such order.

Lexicographical order is defined in following way. When we compare s and t, first we find the leftmost position with differing characters: s_{i} ≠ t_{i}. If there is no such position (i. e. s is a prefix of t or vice versa) the shortest string is less. Otherwise, we compare characters s_{i} and t_{i} according to their order in alphabet.

-----Input-----

The first line contains an integer n (1 ≤ n ≤ 100): number of names.

Each of the following n lines contain one string name_{i} (1 ≤ |name_{i}| ≤ 100), the i-th name. Each name contains only lowercase Latin letters. All names are different.

-----Output-----

If there exists such order of letters that the given names are sorted lexicographically, output any such order as a permutation of characters 'a'–'z' (i. e. first output the first letter of the modified alphabet, then the second, and so on).

Otherwise output a single word "Impossible" (without...

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Give me a Python code to find out any such order?
assistant
#Here is Python code to find out any such order.

#return if sa contains a real prefix: sb
def isPrefix( sa , sb ):
    if len(sa) <= len(sb):
        return False
    return sa[0:len(sb)] == sb

def getOrder( sa , sb ):
    for i in range( 0 , min( len(sa) , len(sb) ) ):
        if sa[i] != sb[i]:
            return sa[i],sb[i]

test = False
if test:
    fp = open("C-4.in","r")
    n = int(fp.readline().strip())
    names = [ fp.readline().strip() for i in range(0,n)]
    fp.close()
else:
    n = int(input().strip())
    names = [ input().strip() for i in range(0,n)]

g = [[False]*26 for i in range(0,26)]

res = True
for i in range(1,n):
    if names[i-1] == names[i] or isPrefix( names[i] , names[i-1] ):
        continue
    elif isPrefix( names[i-1] , names[i] ):
        res = False
        break
    else:
        ca,cb = getOrder( names[i-1] , names[i] )
        #print(ca,"<",cb)
        if g[ord(cb)-ord('a')][ord(ca)-ord('a')]:
            res = False
            break
        else:
            g[ord(ca)-ord('a')][ord(cb)-ord('a')] = True

def printG():
    print("  abcdefghijklmnopqrstuvwxyz")
    for i in range(0,26):
        print( chr( ord("a") + i ) , "".join( [ "1" if x else "0" for x in g[i]] ) , sep = "")
#printG()

if not res:
    print("Impossible")
else:

    def getZeroIndegreeNode():
        for i in range(0,26):
            if not used[i] and indegree[i] == 0:
                return i
        return -1
    #topo sort
    theOrder = []
    indegree = [0] * 26
    used = [False] * 26

    #calc indegree
    for i in range(0,26):
        ithIndegree = 0
        for j in range(0,26):
            if g[j][i]: ithIndegree += 1
        indegree[i] = ithIndegree
        
    for i in range(0,26):
        zeroIndegreeNode = getZeroIndegreeNode()
        if zeroIndegreeNode == -1:
            res = False
            break
        else:
            used[zeroIndegreeNode] = True
            theOrder.append( chr( ord('a') + zeroIndegreeNode ) )
            for j in range(0,26):
                if g[zeroIndegreeNode][j]:
                    indegree[j] -=...