# boj, 1439 : 뒤집기, python
# 그리디 알고리즘
def solution(word):
result = [0,0]
if word[0] == '0':
result[0] += 1
else:
result[1] += 1
for i in range(len(word)-1):
if word[i] != word[i+1]:
if word[i+1] == '0':
result[0] += 1
else:
result[1] += 1
return min(result)
S = list(str(input()))
print(solution(S))