import sys
from collections import Counter

input_str = str(sys.stdin.readline().strip())

s_len = len(input_str)
first_part = input_str[:s_len//2]
sencond_part = input_str[s_len//2:]

def change_need(part):
    max_num =  max(Counter(part).values())
    return len(part) - max_num

print(change_need(first_part) + change_need(sencond_part))