from collections import defaultdict

d = defaultdict(int)

for s in input():
    d[s] += 1
    
ans = ''
for k, v in d.items():
    
    if v == 1:
        ans += k
        
    elif v != 2:
        print('Impossible')
        exit()
        
if len(ans) != 1:
    print('Impossible')
    
else:
    print(ans)