Bishops

The bishop in chess is a piece that attacks all squares on the same diagonal (on both diagonals).

Shakhriyar placed m bishops on a chessboard of size n * n. Now he wants to count the number of squares that are not attacked by bishops. Help Shahriyar in this matter.

Input

First line contains two integers: the size n (1 ≤ n ≤ 10^6) of a chessboard and the number of bishops m (1 ≤ m ≤ 10^5). Each of the next m lines contains pair of integers: r[i] и c[i] (1 ≤ r[i], c[i] ≤ n) - the numbers of row and column where the bishop number i is located. The bishops are numbered from 1 to m. All bishops are located on different squares.

Output

Print the number of squares not attacked by the bishops.

Examples