Four squares of side lengths 5, 8, 11, and 13 units are arranged in increasing size order so that their left and bottom edges align. From smallest to largest the squares alternate in color white, purple, white, purple, as shown. What is the area of the visible purple region, in square units?
四个边长分别为 5、8、11、13 个单位的正方形从小到大依次排列，使得它们的左侧边和底边分别对齐。从小到大，四个正方形的颜色按白色、紫色、白色、紫色交替（如图）。可见紫色区域的面积为多少平方单位？