

import sys

import math

def resolve():

    S=list(input().rstrip())

    c=0

    for i in range(math.floor(len(S)/2)):

        if(S[i]!=S[len(S)-i-1]):

            c+=1

    print(c)



from io import StringIO

import unittest



class TestClass(unittest.TestCase):

    def assertIO(self, input, output):

        stdout, stdin = sys.stdout, sys.stdin

        sys.stdout, sys.stdin = StringIO(), StringIO(input)

        resolve()

        sys.stdout.seek(0)

        out = sys.stdout.read()[:-1]

        sys.stdout, sys.stdin = stdout, stdin

        self.assertEqual(out, output)

    def test_入力例_1(self):

        input = """redcoder"""

        output = """1"""

        self.assertIO(input, output)

    def test_入力例_2(self):

        input = """vvvvvv"""

        output = """0"""

        self.assertIO(input, output)

    def test_入力例_3(self):

        input = """abcdabc"""

        output = """2"""

        self.assertIO(input, output)



if __name__ == "__main__":

    #unittest.main()

    resolve()


import sys

import math

def resolve():

    S=input().rstrip()

    c=0

    for i in range((len(S)//2)):

        if(S[i]!=S[-i-1]):

            c+=1

    print(c)



from io import StringIO

import unittest



class TestClass(unittest.TestCase):

    def assertIO(self, input, output):

        stdout, stdin = sys.stdout, sys.stdin

        sys.stdout, sys.stdin = StringIO(), StringIO(input)

        resolve()

        sys.stdout.seek(0)

        out = sys.stdout.read()[:-1]

        sys.stdout, sys.stdin = stdout, stdin

        self.assertEqual(out, output)

    def test_入力例_1(self):

        input = """redcoder"""

        output = """1"""

        self.assertIO(input, output)

    def test_入力例_2(self):

        input = """vvvvvv"""

        output = """0"""

        self.assertIO(input, output)

    def test_入力例_3(self):

        input = """abcdabc"""

        output = """2"""

        self.assertIO(input, output)



if __name__ == "__main__":

    #unittest.main()

    resolve()
import sys
import math


def resolve():
    S = list(input().rstrip())
    c = 0
    for i in range(math.floor(len(S) / 2)):
        if S[i] != S[len(S) - i - 1]:
            c += 1
    print(c)


from io import StringIO
import unittest


class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)

    def test_入力例_1(self):
        input = """redcoder"""
        output = """1"""
        self.assertIO(input, output)

    def test_入力例_2(self):
        input = """vvvvvv"""
        output = """0"""
        self.assertIO(input, output)

    def test_入力例_3(self):
        input = """abcdabc"""
        output = """2"""
        self.assertIO(input, output)


if __name__ == "__main__":
    # unittest.main()
    resolve()

import sys
import math


def resolve():
    S = input().rstrip()
    c = 0
    for i in range((len(S) // 2)):
        if S[i] != S[-i - 1]:
            c += 1
    print(c)


from io import StringIO
import unittest


class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)

    def test_入力例_1(self):
        input = """redcoder"""
        output = """1"""
        self.assertIO(input, output)

    def test_入力例_2(self):
        input = """vvvvvv"""
        output = """0"""
        self.assertIO(input, output)

    def test_入力例_3(self):
        input = """abcdabc"""
        output = """2"""
        self.assertIO(input, output)


if __name__ == "__main__":
    # unittest.main()
    resolve()

-    S = list(input().rstrip()) +    S = input().rstrip() -    for i in range(math.floor(len(S) / 2)): -        if S[i] != S[len(S) - i - 1]: +    for i in range((len(S) // 2)): +        if S[i] != S[-i - 1]: