from collections import Counter
from string import ascii_lowercase
from functools import lru_cache
import sys
s = input()
sys.setrecursionlimit(100000000)
tmp_c = Counter(s)
val = sum(tmp_c[s_tmp]%2 for s_tmp in ascii_lowercase)
if val==0:
    val=1
if val>5:
    raise
print(val)
