def fish(a: list[int], b: list[int]) -> int:
    size = len(a)
    dead = 0
    downstream: list[int] = []

    for i in range(size):
        if b[i] == 1:
            downstream.append(a[i])
        else:
            while downstream:
                dead += 1
                if a[i] > downstream[-1]:
                    downstream.pop()
                else:
                    break

    return size - dead