from collections import defaultdict
import sys,heapq,bisect,math,itertools,string,queue,datetime
sys.setrecursionlimit(10**8)
INF = float('inf')
mod = 10**9+7
eps = 10**-7
AtoZ = [chr(i) for i in range(65,65+26)]
atoz = [chr(i) for i in range(97,97+26)]

def inpl(): return list(map(int, input().split()))
def inpl_s(): return list(input().split())

S = input()
num = [0]*26

for s in S:
	num[ord(s)-97] += 1

tmp = 0
for n in num:
	if n%2 == 1:
		tmp += 1

print(max(tmp,1))
