import sys





def solve():

    readline = sys.stdin.buffer.readline

    mod = 10 ** 9 + 7

    s = str(readline().rstrip().decode('utf-8'))

    cnt = 0

    for i in range(len(s)):

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

            cnt += 1

    print((cnt // 2))





if __name__ == '__main__':

    solve()

import sys





def solve():

    input = sys.stdin.readline

    mod = 10 ** 9 + 7

    s = str(input().rstrip('\n'))

    cnt = 0

    for i in range(len(s)):

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

            cnt += 1

    print((cnt // 2))





if __name__ == '__main__':

    solve()

import sys


def solve():
    readline = sys.stdin.buffer.readline
    mod = 10**9 + 7
    s = str(readline().rstrip().decode("utf-8"))
    cnt = 0
    for i in range(len(s)):
        if s[i] != s[-i - 1]:
            cnt += 1
    print((cnt // 2))


if __name__ == "__main__":
    solve()

import sys


def solve():
    input = sys.stdin.readline
    mod = 10**9 + 7
    s = str(input().rstrip("\n"))
    cnt = 0
    for i in range(len(s)):
        if s[i] != s[-i - 1]:
            cnt += 1
    print((cnt // 2))


if __name__ == "__main__":
    solve()

-    readline = sys.stdin.buffer.readline +    input = sys.stdin.readline -    s = str(readline().rstrip().decode("utf-8")) +    s = str(input().rstrip("\n"))