s = input().strip()
counts = [0] * 26

for c in s:
    counts[ord(c) - ord('a')] += 1

# Check if any character appears more than twice
if any(c > 2 for c in counts):
    print("Impossible")
else:
    k = sum(1 for c in counts if c == 2)
    m = sum(1 for c in counts if c == 1)
    total_non_zero = k + m
    
    if k == 6 and m == 1 and total_non_zero == 7:
        # Find the character that occurs once
        for i in range(26):
            if counts[i] == 1:
                print(chr(ord('a') + i))
                break
    else:
        print("Impossible")