def derivative(xs: list):
    """ xs represent coefficients of a polynomial.
    xs[0] + xs[1] * x + xs[2] * x^2 + ....
     Return derivative of this polynomial in the same form.
    >>> derivative([3, 1, 2, 4, 5])
    [1, 4, 12, 20]
    >>> derivative([1, 2, 3])
    [2, 6]
    
    Example solution:
    # line 1
    derivative = []
    # line 2
    for i, x in enumerate(xs):
        # line 3
        derivative.append(i * x)
    # line 4
    return derivative
    
    """
    # Please print out which line of the above program contains an error. E.g. if the bug is on line 4 then print 4
    # END OF CONTEXT
    print("4")
    # END OF SOLUTION

def check(candidate):

    import io
    from contextlib import redirect_stdout

    f = io.StringIO()
    with redirect_stdout(f):
        candidate([])
    out = f.getvalue().strip('\n')

    assert "4" == out
    for i in range(0, 10):
        if i != 4:
            assert str(i) != out

if __name__ == '__main__':
    check(derivative)
