550 B | 550 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
s = input()
d = {}
for i in s:
d[i] = d.get(i, 0) + 1
odd_cnt = 0
for v in d.values():
if v % 2 == 1:
odd_cnt += 1
print(max(odd_cnt - 1, 0))
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()