#include <bits/stdc++.h>
using namespace std;
const int N = 100002;
int n, m, l, r, p, t, k, sum, tot, cnt, a[N], b[N], c[N], h, now, vis[N],
    Vis[N];
long long ans;
inline int read() {
  register int x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  return x * f;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    scanf("%d%d", &p, &k);
    vis[p] = Vis[k] = 1;
  }
  for (int i = 2; i < n; i++) {
    if (vis[i] == 0) ans++;
    if (Vis[i] == 0) ans++;
  }
  if ((n & 1) && (!vis[n / 2 + 1]) && (!Vis[n / 2 + 1])) ans--;
  printf("%d\n", ans);
}
