inputs = input()

i_dict ={}

inputs = list(inputs)

for i in inputs:
    if i not in i_dict:
        i_dict[i] = 1
    else:
        i_dict[i] += 1

for k, v in i_dict.items():
    i_dict[k] = v % 2
    
ans = 0
for v in i_dict.values():
    ans += v

if ans == 0:
    print(1)
else:
    print(ans)