import sys
input = sys.stdin.readline

def main():
    N = int(input())
    S = input().strip()

    # dp[i]: 先頭 i 文字で残せる '0' の最大数
    dp = [0] * (N + 1)
    for i in range(1, N + 1):
        if S[i-1] == '1':
            dp[i] = dp[i-1]
        else:  # S[i-1] == '0'
            keep = (dp[i-3] if i >= 3 else 0) + 1
            dp[i] = max(dp[i-1], keep)

    total_zeros = S.count('0')
    ans = total_zeros - dp[N]
    print(ans)

if __name__ == "__main__":
    main()