#include <bits/stdc++.h>
using namespace std;
int a[200000][2];
bool s[2000], t[2000], mmark[2000], nmark[2000];
int main() {
  long long n, m, ans = 0;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    cin >> a[i][0] >> a[i][1];
    mmark[a[i][0]] = nmark[a[i][1]] = 1;
  }
  for (int i = 2; i < n; i++)
    if (!nmark[i] && !t[i]) {
      t[i] = 1;
      ans++;
    }
  for (int i = 2; i < n; i++)
    if (!mmark[i] && !s[i]) {
      s[i] = 1;
      ans++;
    }
  if (n % 2 && nmark[n / 2 + 1] == 0 && mmark[n / 2 + 1] == 0) ans--;
  cout << ans << endl;
}
