import functools

alpha = "abcdefghijklmnopqrstuvwxyz"
def check(s):
    dict = {w:0 for w in alpha}
    for c in s:
        if dict[c]==1:
            dict[c]=0
            continue
        if dict[c]==0:
            dict[c]=1
    aaaa = [dict[c] for c in alpha]
    return functools.reduce(lambda a,b:a+b,aaaa)

def main():
    n = check(input())
    print(max(1,n))

    # total = functools.reduce(lambda a, b: a * (2 if b % 2 == 1 else 1),A)
    # print(pow(3 ,len(A)-1)*total)
    # # print(123)
    # b = [int(a) for a in input().split(" ")]
    # print(b)
    # [input() for a in range(3)]


if __name__ == "__main__":
    main()
