#include <bits/stdc++.h>
using namespace std;
const int maxn = 1003;
bool a[maxn], b[maxn];
inline int get() {
  int f = 0, v = 0;
  char ch;
  while (!isdigit(ch = getchar()))
    if (ch == '-') break;
  if (ch == '-')
    f = 1;
  else
    v = ch - 48;
  while (isdigit(ch = getchar())) v = v * 10 + ch - 48;
  if (f == 1)
    return -v;
  else
    return v;
}
inline double getd() {
  double d = 0, d2 = 0, d3 = 1;
  char ch;
  bool flag = 0;
  while (!isdigit(ch = getchar()))
    if (ch == '-') break;
  if (ch == '-')
    flag = true;
  else
    d = ch - 48;
  while (isdigit(ch = getchar())) d = d * 10 + ch - 48;
  if (ch == '.') {
    while (isdigit(ch = getchar())) d2 = d2 * 10 + ch - 48, d3 = d3 * 0.1;
    d += d3 * d2;
  }
  if (flag)
    return -d;
  else
    return d;
}
inline void write(int x) {
  int _ = x, len = 0, p[20];
  if (_ < 0) putchar('-'), _ = -_;
  while (_) p[++len] = _ % 10, _ /= 10;
  if (!x) p[++len] = 0;
  while (len) putchar(p[len--] + '0');
}
inline void writeln(int x) {
  write(x);
  puts("");
}
int main() {
  int n = get(), m = get();
  for (int i = 1; i <= m; i++) a[get()] = 1, b[get()] = 1;
  int ans = 0;
  for (int i = 2; i < n; i++) ans += !a[i], ans += !b[i];
  m = n / 2 + 1;
  if (n % 2 == 1 && !a[m] && !b[m]) ans--;
  cout << ans << endl;
  return 0;
}
