#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;
  }
  int prev[n];
  int uniq[n];
  int next[n];
  for (int i = 0; i < n; ++i) {
    prev[i] = 0;
    uniq[i] = 0;
    next[i] = 0;
  }
  for (int i = 0; i < k; i++) {
    uniq[quest[i]] = 1;
    if (uniq[quest[i] - 1] == 1) next[quest[i] - 1] = 1;
    if (uniq[quest[i] + 1] == 1) prev[quest[i] + 1] = 1;
  }
  int answer = 3 * n - 2;
  for (int i = 0; i < n; ++i) {
    if (uniq[i] == 1) answer -= 1;
    if (i > 0) {
      if (uniq[i] + next[i - 1] == 2) answer -= 1;
    }
    if (i < n - 1) {
      if (uniq[i] + prev[i + 1] > 1) answer -= 1;
    }
  }
  cout << answer << endl;
}
