#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int k;
  cin >> n;
  cin >> k;
  int quest[k];
  int q;
  for (int i = 0; i < k; ++i) {
    cin >> q;
    quest[i] = q - 1;
  }
  bool prev[n];
  bool uniq[n];
  bool next[n];
  for (int i = 0; i < n; ++i) {
    prev[i] = false;
    uniq[i] = false;
    next[i] = false;
  }
  for (int i = 0; i < k; i++) {
    uniq[quest[i]] = true;
    if (uniq[quest[i] - 1]) next[quest[i] - 1] = true;
    if (uniq[quest[i] + 1]) prev[quest[i] + 1] = true;
  }
  int answer = 3 * n - 2;
  for (int i = 0; i < n; ++i) {
    if (uniq[i]) answer -= 1;
    if (i > 0) {
      if (uniq[i] & next[i - 1]) answer -= 1;
    }
    if (i < n - 1) {
      if (uniq[i] & prev[i + 1]) answer -= 1;
    }
  }
  cout << answer << endl;
}
