def skip(s,n):
    """
    Returns a copy of s, only including positions that are multiples of n

    A position is a multiple of n if pos % n == 0.

    Examples:
        skip('hello world',1) returns 'hello world'
        skip('hello world',2) returns 'hlowrd'
        skip('hello world',3) returns 'hlwl'
        skip('hello world',4) returns 'hor'

    Parameter s: the string to copy
    Precondition: s is a nonempty string

    Parameter n: the letter positions to accept
    Precondition: n is an int > 0
    """
    assert type(s) == str and len(s) != 0
    assert type(n) == int and n > 0

    s2 = ''
    idx = 0

    for element in s:
        if idx % n == 0:
            s2 = s[idx] + s2
        idx += 1
    return s2[::-1]
    # pass