def main():
    S = input()

    str_ = dict()
    for s in S:
        str_[s] = str_.get(s, 0) + 1

    even = 0
    odd = 0
    for key, val in str_.items():
        if val % 2:
            odd += 1
        else:
            even += 1

    if odd <= 1:
        print(1)
    elif even == 0:
        print(odd)
    else:
        print(odd)

main()
