#0002A3

Pruge

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

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


InputThe first and only line of input contains four integers X1, Y1, X2 and 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.

OutputThe 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++).


Input

2 3 7 6

Output

8

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