def min_operations_to_good_string(S):
    N = len(S)
    ones = S.count('1')
    zero_blocks = []
    i = 0

    while i < N:
        if S[i] == '0':
            start = i
            while i < N and S[i] == '0':
                i += 1
            length = i - start
            if length >= 2:
                zero_blocks.append(length)
        else:
            i += 1

    total_zeros_in_blocks = sum(zero_blocks)
    if total_zeros_in_blocks < ones:
        return 0

    zero_blocks.sort(reverse=True)
    ops = 0
    for block in zero_blocks:
        for _ in range(block):
            total_zeros_in_blocks -= 1
            ops += 1
            ones += 1
            if total_zeros_in_blocks < ones:
                return ops
    return ops

if __name__ == "__main__":
    N = int(input())
    S = input().strip()
    print(min_operations_to_good_string(S))