#include <bits/stdc++.h>
using namespace std;
int a[1005], b[1005];
int main() {
  int n, m, x, y, ans = 0;
  scanf("%d%d", &n, &m);
  while (m--) {
    scanf("%d%d", &x, &y);
    a[x] = 1, b[y] = 1;
  }
  for (int i = 2; i < n; i++) {
    ans += !a[i] + !b[i];
    if (n % 2 && (n + 1) / 2 == i) ans -= (!a[i] + !b[i]) / 2;
  }
  printf("%d\n", ans);
}
