import sys
input = lambda : sys.stdin.readline().rstrip()
sys.setrecursionlimit(max(1000, 10**9))
write = lambda x: sys.stdout.write(x+"\n")


s = input()
n = len(s)
from collections import Counter
c = Counter(s)
ans = sum(v%2==1 for v in c.values())
print(ans)
# d = {k:0 for k in c.keys()}
# e = [0]*n # 奇数回数の文字の最後の出現位置
# e2 = [0]*n
# for i,item in enumerate(s):
#     d[item] += 1
#     if d[item]%2==1:
#         e2[i] = 1
#     else:
#         e2[i] = -1
#     if c[item]%2==1 and d[item]==c[item]:
#         e[i] = 1
        
# ans = 0
# val = 0
# for i in range(n):
#     if val+e[i]>=2:
#         ans += 1
#         val = 1
#     else:
#         val += e[i]
# print(ans+1)