def min_operations_to_good_string(S):
    balance = 0
    flips = 0
    for ch in S:
        if ch == '1':
            balance += 1
        else:
            balance -= 1
        if balance < 0:
            flips += 1
            balance += 2  # simulate flipping this 0 to 1
    return flips

# Example:
S = input().strip()
print(min_operations_to_good_string(S))