CF1355C Count Triangles

给出四个整数$A,B,C,D$ ，其中$1 \leq A \leq B \leq C \leq D \leq 5 \cdot 10^5$ 求三边长分别为 $x,y,z$ 并且满足 $A \leq x \leq B \leq y \leq C \leq z \leq D$ 的非退化三角形（三顶点不共线）的个数

一行四个整数： $A,B,C,D$。

一行，输出满足条件的三角形个数。

In the first example Yuri can make up triangles with sides $ (1, 3, 3) $ , $ (2, 2, 3) $ , $ (2, 3, 3) $ and $ (2, 3, 4) $ . In the second example Yuri can make up triangles with sides $ (1, 2, 2) $ , $ (2, 2, 2) $ and $ (2, 2, 3) $ . In the third example Yuri can make up only one equilateral triangle with sides equal to $ 5 \cdot 10^5 $ .