import sys

def min_flips_to_good(S: str) -> int:
    total = S.count('0')
    last = -10
    cnt = 0
    for i, c in enumerate(S):
        if c == '0' and i - last >= 3:
            cnt += 1
            last = i
    return total - cnt

def main():
    input = sys.stdin.readline
    n = int(input().strip())
    S = input().strip()
    print(min_flips_to_good(S))

if __name__ == "__main__":
    main()