103543: [Atcoder]ABC354 D - AtCoder Wallpaper

Problem Statement

The pattern of AtCoder's wallpaper can be represented on the $xy$-plane as follows:

• The plane is divided by the following three types of lines:
  • $x = n$ (where $n$ is an integer)
  • $y = n$ (where $n$ is an even number)
  • $x + y = n$ (where $n$ is an even number)
• Each region is painted black or white. Any two regions adjacent along one of these lines are painted in different colors.
• The region containing $(0.5, 0.5)$ is painted black.

The following figure shows a part of the pattern.

You are given integers $A, B, C, D$. Consider a rectangle whose sides are parallel to the $x$- and $y$-axes, with its bottom-left vertex at $(A, B)$ and its top-right vertex at $(C, D)$. Calculate the area of the regions painted black inside this rectangle, and print twice that area.

It can be proved that the output value will be an integer.

Constraints

• $-10^9 \leq A, B, C, D \leq 10^9$
• $A < C$ and $B < D$.
• All input values are integers.

Input

The input is given from Standard Input in the following format:

$A$ $B$ $C$ $D$

Output

Print the answer on a single line.

Sample Input 1

0 0 3 3

Sample Output 1

10

We are to find the area of the black-painted region inside the following square:

The area is $5$, so print twice that value: $10$.

Sample Input 2

-1 -2 1 3

Sample Output 2

11

The area is $5.5$, which is not an integer, but the output value is an integer.

Sample Input 3

-1000000000 -1000000000 1000000000 1000000000

Sample Output 3

4000000000000000000

This is the case with the largest rectangle, where the output still fits into a 64-bit signed integer.