It is symmetrical across the diagonal so we will only look at the top left quarter, then multiply by 2 when we are done.
Because it is 1/4 of the rectangle it has area 1/4 of the original.
1/4 x 8x5 = 10.
This leaves two right triangles and the grey area.
The top right triangle has a base of (8/2)-1=3
And height of 5/2=2.5
Area = 1/2 x 3 x 2.5 = 3.75
The bottom left triangle has height of (5/2)-1=1.5
And base of 8/2=4
Area = 1/2 x 1.5 x 4 = 3
Top left grey area is the area of the the quarter minus the areas of the two right triangles = 10 - 3.75 - 3 = 3.25
There are two of these gray areas so final answer is 2 x 3.25 = 6.5