622 B | 622 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def f(s, k):
ret = ""
for i in s:
ret += chr((ord(i) - 97 - k + 26) % 26 + 97)
if "i" in ret:
return 0
return 1
def solve():
s = input()
ans = 0
for i in range(26):
ans += f(s, i)
if ans == 0:
return "impossible"
return ans
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()
p(ret)