Let P be the point on the parabola y = x² such that the slope of the tangent to the parabola at the point P is 4. Let Q be the point in the first quadrant lying on the circle x² + y² = 2 such that the slope of the tangent to the circle at the point Q is −1. Let R be the point in the first quadrant lying on the ellipse x² + 4y² = 8 such that the slope of the tangent to the ellipse at the point R is −1/2. Then the radius of the circle passing through the points P, Q and R is