#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, k;
  cin >> n >> k;
  set<pair<int, int>> s;
  vector<bool> vis(n + 1);
  vector<int> x(k);
  s.insert({1, 1});
  s.insert({n, n});
  int i, j, p, q;
  if (2 <= n) s.insert({1, 2});
  if (n - 1 > 0) s.insert({n, n - 1});
  for (i = 2; i < n; i++)
    s.insert({i, i}), s.insert({i, i - 1}), s.insert({i, i + 1});
  for (i = 0; i < k; i++) cin >> x[i];
  reverse(x.begin(), x.end());
  set<pair<int, int>>::iterator it;
  for (i = 0; i < k; i++) {
    it = s.find({x[i], x[i]});
    if (it != s.end()) s.erase(it);
    if (vis[x[i] - 1]) {
      it = s.find({x[i], x[i] - 1});
      if (it != s.end()) s.erase(it);
    }
    if (vis[x[i] + 1]) {
      it = s.find({x[i], x[i] + 1});
      if (it != s.end()) s.erase(it);
    }
    vis[x[i]] = 1;
  }
  cout << s.size() << endl;
  return 0;
}
