pruge

문제

A pattern composed of dark stripes (as depicted in the figure below) spans the entire first quadrant in the rectangular coordinate system.

[이미지 1]

Write a program that finds the total number of dark squares inside the given rectangle. 

입력

The first and only line of input contains four integers X1, Y1, X2, Y2 – the coordinates of the rectangle with the lower left corner in (X1,Y1) and upper right corner in (X2,Y2). 

0 ≤ X1 < X2 ≤ 1 000 000, 0 ≤ Y1 < Y2 ≤ 1 000 000.

출력

The first and only line of output should contain a single integer – the total number of dark squares inside the given rectangle.

Note: use the 64-bit signed integer type (int64 in Pascal, long long in C/C++).