Problem Statement

We have a grid with H horizontal rows and W vertical columns. Let Square (i,j) be the square at the i-th row and j-th column.
There are M obstacles on this grid. The i-th obstacle is at Square (X_i, Y_i).
We have a rook, the chess piece, on Square (1, 1). In one move, it can move to the right or downward through any number of squares without obstacles.
Find the number of squares the rook can reach in two moves or less.

Input

Input is given from Standard Input in the following format:
H W M
X_1 Y_1
$\vdots$
X_M Y_M

Output

Print the number of squares the rook can reach in two moves or less.

Constraints

• $1\leq H,W \leq 2\times 10^5$
• $0\leq M \leq 2\times 10^5$
• $1\leq X_i \leq H$
• $1\leq Y_i \leq W$
• $(X_i,Y_i) \neq (1,1)$
• (X_i,Y_i) are distinct.
• All values in input are integers.

Sample Input 1

4 3 2
2 2
3 3

Sample Output 1

10

Every square without an obstacle can be reached in two moves or less.

Sample Input 2

5 4 4
3 2
3 4
4 2
5 2

Sample Output 2

14

Every square without an obstacle except (4,4) and (5,4) can be reached in two moves or less.

Sample Input 3

200000 200000 0

Sample Output 3

40000000000

见题面 Input 部分

见题面 Output 部分