# https://yukicoder.me/problems/no/3774



def main():
    S = input()

    N = int(len(S))

    left = 0
    right = N -1
    answer = 0
    while left < right:
        if S[left] != S[right]:
            answer += 1
        left += 1
        right -= 1
    print(answer)









if __name__ == "__main__":
    main()