s = input()
m = {}
for c in s:
    if not c in m: m[c] = 1
    else: m[c] += 1

ok = True
flag = True
for v in m:
    #print(v)
    #print(m[v])
    if (m[v] < 2):
        if (flag == True and m[v] == 1):
            res = v
            flag = False
        else: ok = False

if (ok and len(m) == 7): print(res)
else: print("Impossible")