import math
import sys
def S(): return sys.stdin.readline().rstrip()
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def LS(): return list(sys.stdin.readline().rstrip().split())

s = S()
s_set = set(list(s))
if len(s_set) != 7:
    print("Impossible")
else:
    flag = 0
    for i in s_set:
        if s.count(i) == 2:
            continue
        elif s.count(i) > 2:
            print("Impossible")
            break
        elif s.count(i) == 1:
            if flag == 1:
                print("Impossible")
                break
            else:
                ans = i
                flag = 1
    else:
        print(ans)