#!/usr/bin/python



import sys



def solve(s):

    count = 0

    len_s = len(s)

    l = len_s // 2

    for i in range(l):

        if s[i] != s[len_s -1 -i]:

            count += 1



    print(count)



if __name__ == '__main__':

    s = sys.stdin.readline().strip()

    solve(s)
import sys



S = sys.stdin.readline().strip()

ls = len(S)



ans = 0

h = ls // 2

for i in range(h):

    if S[i] != S[ls - 1 - i]:

        ans += 1



print(ans)
#!/usr/bin/python
import sys


def solve(s):
    count = 0
    len_s = len(s)
    l = len_s // 2
    for i in range(l):
        if s[i] != s[len_s - 1 - i]:
            count += 1
    print(count)


if __name__ == "__main__":
    s = sys.stdin.readline().strip()
    solve(s)

import sys

S = sys.stdin.readline().strip()
ls = len(S)
ans = 0
h = ls // 2
for i in range(h):
    if S[i] != S[ls - 1 - i]:
        ans += 1
print(ans)

-#!/usr/bin/python - -def solve(s): -    count = 0 -    len_s = len(s) -    l = len_s // 2 -    for i in range(l): -        if s[i] != s[len_s - 1 - i]: -            count += 1 -    print(count) - - -if __name__ == "__main__": -    s = sys.stdin.readline().strip() -    solve(s) +S = sys.stdin.readline().strip() +ls = len(S) +ans = 0 +h = ls // 2 +for i in range(h): +    if S[i] != S[ls - 1 - i]: +        ans += 1 +print(ans)