#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  int answer = 3 * n - 2;
  unordered_set<int> present;
  unordered_set<int> present_set[n + 1];
  int temp1;
  for (int i = 0; i < k; i++) {
    cin >> temp1;
    if (temp1 != 1 && !present_set[temp1 - 1].count(temp1) &&
        present.count(temp1 - 1)) {
      answer--;
      present_set[temp1 - 1].insert(temp1);
    }
    if (temp1 != n && !present_set[temp1 + 1].count(temp1) &&
        present.count(temp1 + 1)) {
      answer--;
      present_set[temp1 + 1].insert(temp1);
    }
    present.insert(temp1);
  }
  cout << answer - present.size();
}
