# -*- coding: utf-8 -*-
import sys

def main() -> None:
    N = int(sys.stdin.readline())
    S = sys.stdin.readline().strip()

    last_zero_kept = -3
    ops = 0

    for i, ch in enumerate(S):
        if ch == '0':
            if i - last_zero_kept >= 3:
                last_zero_kept = i
            else:
                ops += 1

    print(ops)

if __name__ == "__main__":
    main()