B"Gerald plays the following game. He has a checkered field of size n xe2 x80 x89 xc3 x97 xe2 x80 x89n cells, where m various cells are banned. Before the game, he has to put a few chips on some border (but not corner) board cells. Then for n xe2 x80 x89- xe2 x80 x891 minutes, Gerald every minute moves each chip into an adjacent cell. He moves each chip from its original edge to the opposite edge. Gerald loses in this game in each of the three cases: In that case he loses and earns 0 points. When nothing like that happened, he wins and earns the number of points equal to the number of chips he managed to put on the board. Help Gerald earn the most points. The first line contains two space-separated integers n and m (2 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n xe2 x80 x89 xe2 x89 xa4 xe2 x80 x891000, 0 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89m xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89105) -- the size of the field and the number of banned cells. Next m lines each contain two space-separated integers. Specifically, the i-th of these lines contains numbers xi and yi (1 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89xi, xe2 x80 x89yi xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n) -- the coordinates of the i-th banned cell. All given cells are distinct. Consider the field rows numbered from top to bottom from 1 to n, and the columns -- from left to right from 1 to n. Print a single integer -- the maximum points Gerald can earn in this game. In the first test the answer equals zero as we can't put chips into the corner cells. In the second sample we can place one chip into either cell (1, 2), or cell (3, 2), or cell (2, 1), or cell (2, 3). We cannot place two chips. In the third sample we can only place one chip into either cell (2, 1), or cell (2, 4)."...