#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, x;
  while (cin >> n >> k) {
    map<int, set<int>> mp;
    for (int i = 1; i <= k; i++) {
      cin >> x;
      if (!mp.count(x)) mp[x] = set<int>();
      if (x > 1 && mp.count(x - 1)) mp[x - 1].insert(x);
      if (x < n && mp.count(x + 1)) mp[x + 1].insert(x);
    }
    if (n == 1) {
      cout << 0 << endl;
      continue;
    }
    int ans = 0;
    for (int i = 1; i <= n; i++) {
      if (!mp.count(i))
        ans += (i == 1 || i == n ? 2 : 3);
      else if (i == 1 || i == n)
        ans += 1 - mp[i].size();
      else
        ans += 2 - mp[i].size();
    }
    cout << ans << endl;
  }
  return 0;
}
