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