import bisect

n = int(input())
s = input().strip()
pos = [i for i in range(n) if s[i] == '0']
m = len(pos)

if m <= 1:
    print(0)
else:
    dp = [0] * m
    dp[0] = 1
    for i in range(1, m):
        target = pos[i] - 3
        j = bisect.bisect_right(pos, target) - 1
        if j >= 0:
            current = dp[j] + 1
        else:
            current = 1
        dp[i] = max(dp[i-1], current)
    print(m - dp[-1])