def main():

    import sys

    input = sys.stdin.readline

    sys.setrecursionlimit(10**7)

    from collections import Counter, deque

    #from collections import defaultdict

    from itertools import combinations, permutations, accumulate, groupby

    #from itertools import product

    from bisect import bisect_left,bisect_right

    from heapq import heapify, heappop, heappush

    from math import floor, ceil

    #from operator import itemgetter



    #inf = 10**17

    #mod = 10**9 + 7



    s = input().rstrip()

    r = len(s)

    res = 0

    for i in range(r//2):

        if s[i] != s[-(i+1)]:

            res += 1

    print(res)







if __name__ == '__main__':

    main()
def main():

    import sys

    input = sys.stdin.readline

    sys.setrecursionlimit(10**7)

    from collections import Counter, deque

    #from collections import defaultdict

    from itertools import combinations, permutations, accumulate, groupby

    #from itertools import product

    from bisect import bisect_left,bisect_right

    from heapq import heapify, heappop, heappush

    from math import floor, ceil

    #from operator import itemgetter



    #inf = 10**17

    #mod = 10**9 + 7



    s = input().rstrip()

    res = 0

    ss = s[::-1]

    for i in range(len(s)):

        if s[i] != ss[i]:

            res += 1

    print((res//2))



if __name__ == '__main__':

    main()
def main():
    import sys

    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque

    # from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby

    # from itertools import product
    from bisect import bisect_left, bisect_right
    from heapq import heapify, heappop, heappush
    from math import floor, ceil

    # from operator import itemgetter
    # inf = 10**17
    # mod = 10**9 + 7
    s = input().rstrip()
    r = len(s)
    res = 0
    for i in range(r // 2):
        if s[i] != s[-(i + 1)]:
            res += 1
    print(res)


if __name__ == "__main__":
    main()

def main():
    import sys

    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque

    # from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby

    # from itertools import product
    from bisect import bisect_left, bisect_right
    from heapq import heapify, heappop, heappush
    from math import floor, ceil

    # from operator import itemgetter
    # inf = 10**17
    # mod = 10**9 + 7
    s = input().rstrip()
    res = 0
    ss = s[::-1]
    for i in range(len(s)):
        if s[i] != ss[i]:
            res += 1
    print((res // 2))


if __name__ == "__main__":
    main()

-    r = len(s) -    for i in range(r // 2): -        if s[i] != s[-(i + 1)]: +    ss = s[::-1] +    for i in range(len(s)): +        if s[i] != ss[i]: -    print(res) +    print((res // 2))