import sys


def input():
    return sys.stdin.readline().strip()


def mapint():
    return map(int, input().split())


sys.setrecursionlimit(10 ** 9)
S = list(str(input()))
length = len(S)
cnt = 0
for a, b in zip(S, S[::-1]):
    if a != b:
        cnt += 1
print(cnt // 2)

cc4efa57d33ba1638db8afb4c64495b907e2825b
<mask token>


def input():
    return sys.stdin.readline().strip()


def mapint():
    return map(int, input().split())


<mask token>

<mask token>


def input():
    return sys.stdin.readline().strip()


def mapint():
    return map(int, input().split())


sys.setrecursionlimit(10 ** 9)
<mask token>
for a, b in zip(S, S[::-1]):
    if a != b:
        cnt += 1
print(cnt // 2)

<mask token>


def input():
    return sys.stdin.readline().strip()


def mapint():
    return map(int, input().split())


sys.setrecursionlimit(10 ** 9)
S = list(str(input()))
length = len(S)
cnt = 0
for a, b in zip(S, S[::-1]):
    if a != b:
        cnt += 1
print(cnt // 2)

import sys


def input():
    return sys.stdin.readline().strip()


def mapint():
    return map(int, input().split())


sys.setrecursionlimit(10 ** 9)
S = list(str(input()))
length = len(S)
cnt = 0
for a, b in zip(S, S[::-1]):
    if a != b:
        cnt += 1
print(cnt // 2)
