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 quotes).


-----Examples-----
Input
3
rivest
shamir
adleman

Output
bcdefghijklmnopqrsatuvwxyz

Input
10
tourist
petr
wjmzbmr
yeputons
vepifanov
scottwu
oooooooooooooooo
subscriber
rowdark
tankengineer

Output
Impossible

Input
10
petr
egor
endagorion
feferivan
ilovetanyaromanova
kostka
dmitriyh
maratsnowbear
bredorjaguarturnik
cgyforever

Output
aghjlnopefikdmbcqrstuvwxyz

Input
7
car
care
careful
carefully
becarefuldontforgetsomething
otherwiseyouwillbehacked
goodluck

Output
acbdefhijklmnogpqrstuvwxyz

Difficulty: interview

Starter code:

Solutions:
#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] -= 1

    if not res:
        print("Impossible")
    else:
        print( "".join( theOrder ) )


#import sys
#sys.stdin = open('in.txt')
#sys.setrecursionlimit(10000)
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("in.txt", '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] -= 1
    if not res:
        print("Impossible")
    else:
        print("".join(theOrder))

#import sys
#sys.stdin = open('in.txt')
#sys.setrecursionlimit(10000)
def isPrefix(sa, sb):
    if len(sa) >= len(sb):
        return False
    return sa == sb[0:len(sa)]

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

test = False
if test:
    fp = open('in.txt', 'r')
    n = int(fp.readline().strip())
    names = [fp.readline().strip() for _ in range(n)]
    fp.close()
else:
    n = int(input().strip())
    names = [input().strip() for _ in range(n)]

res = True
g = [[False] * 26 for _ in range(26)]
"""
for i in range(26):
    for j in range(26):
        g[i][j] = False
"""
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 = 

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

if not res:
    print("Impossible")
else:
    def getZeroIndegreeNode():
       for i in range(26):
          if not vis[i] and Indegree[i] == 0:
              return i
       return -1
    
    #cacl Indegree
    strOrder = []
    vis = [False] * 26
    Indegree = [0] * 26
    for i in range(26):
        ithIndegree = 0
        for j in range(26):
            if g[j][i]: ithIndegree += 1
        Indegree[i] = ithIndegree
    
    #get the order string
    for i in range(26):
        ZeroIndegreeNode = getZeroIndegreeNode()
        if ZeroIndegreeNode == -1:
            res = False
            break
        else:
            strOrder.append(chr(ord('a') + ZeroIndegreeNode))
            vis[ZeroIndegreeNode] = True
            for i in range(26):
                if g[ZeroIndegreeNode][i]:
                    Indegree[i] -= 1
    if not res:
        print("Impossible")
    else:
        print("".join(strOrder))


import string

def make_link(G, char1, char2):
    if char1 in G:
        if char2 in G[char1]:
            return
        else:
            G[char1].append(char2)
    else:
        G[char1] = [char2]

def make_order(G, str1, str2):
    for i in range(min(len(str1), len(str2))):
        if str1[i] != str2[i]:
            make_link(G, str1[i], str2[i])
            return True

    if len(str2) > len(str1):
        return True
    else:
        return False

def topological_sort(G, ans, k = '', visited = {}):
    if visited.get(k, True) == False:
        return True
    if k not in visited:
        visited[k] = False
        for next in G.get(k, []):
            if topological_sort(G, ans, next, visited):
                return True
        visited[k] = True
        ans.insert(0, k)
    return False

def main():
    N = int(input())

    strs = [input() for i in range(N)]

    G = {}

    for i in range(N-1):
        if not make_order(G, strs[i], strs[i + 1]):
            print("Impossible")
            return

    G[''] = reversed(string.ascii_lowercase)

    ans = []
    if topological_sort(G, ans):
        print("Impossible")
    else:
        print(''.join(ans))

main()

n=int(input())
x=input()
a=len(x)
lex=dict()
ALPHA=list('abcdefghijklmnopqrstuvwxyz')

for i in ALPHA:
    for j in ALPHA:
        lex[(i,j)]=0
    
for i in range(n-1):
    y=input()
    b=len(y)
    for j in range(min([a,b])):
        if x[j]!=y[j]:
            if lex[(x[j],y[j])]==-1:
                print('Impossible')
                raise SystemExit
            else:
                lex[(x[j],y[j])]=1
                lex[(y[j],x[j])]=-1
                x=y
                a=b
            break
        if j==(min([a,b])-1):
            if a>b:
                print('Impossible')
                raise SystemExit
            x=y
            a=b
    
def biggest(alphabet):
    for i in alphabet:
        t=0
        for j in alphabet:
            if lex[(i,j)]<0:
                t=1
                break
        if t==0:
            return i
    return -1
newalpha=['']*26 
i=0
while ALPHA!=[]:
    BIG=biggest(ALPHA)
    if biggest(ALPHA)==-1:
        print('Impossible')
        raise SystemExit
    newalpha[i]=BIG
    i=i+1
    ALPHA.remove(BIG)

print(''.join(newalpha))