The window is half a circle. We know that the perimeter of a full circle is:

Where D is the diameter of the circle. Since we only have half of the circle the perimeter of our window will be:

We need to find the diameter of the circle, from the picture we conclude the diameter is 40 cm (since the diameter is twice the radius).
Then our perimeter is:

Therefore the perimeter of the window is 62.8 cm.