n = int(input())
s = input().strip()

total_zeros = s.count('0')

if total_zeros == 0:
    print(0)
    exit()

dp = [0] * n
max_val = 0

# Initialize the first element
if s[0] == '0':
    dp[0] = 1
else:
    dp[0] = 0

for i in range(1, n):
    # Update max_val if necessary
    if i >= 3:
        prev_pos = i - 3
        if dp[prev_pos] > max_val:
            max_val = dp[prev_pos]
    # Calculate dp[i]
    if s[i] == '0':
        option = max_val + 1
        dp[i] = max(dp[i-1], option)
    else:
        dp[i] = dp[i-1]

max_keep = dp[-1]
print(total_zeros - max_keep)