s = input()
# ans = 0
# while '01' in s or '10' in s:
#     # print('yes')
#     if '01' in s:
#         s = s.replace('01', '', 1)
#         ans += 2
#     if '10' in s:
#         s = s.replace('10', '', 1)
#         ans += 2
#     # print(s, ans)

# # print(ans)

ans = min(s.count('0'), s.count('1'))
print(ans)