#include <bits/stdc++.h>
using namespace std;
int a[1010], b[1010];
int main() {
  int n, m, i, ans, x, y;
  cin >> n >> m;
  for (i = 1; i <= m; i++) {
    cin >> x >> y;
    a[x] = b[y] = 1;
  }
  for (ans = 0, i = 2; i < n; i++) {
    if (a[i] + b[i] == 1) ans++;
    if (a[i] + b[i] == 0) {
      ans += 2;
      if (n % 2 == 1 && i == (n + 1) / 2) ans--;
    }
  }
  cout << ans << endl;
  return 0;
}
