#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
long int mod = 1e9 + 7;
long int i, j, n, t, m, k;
vector<pair<long int, long int> > a, b;
set<long int> c;
bool cmp(const pair<long int, long int> &l, const pair<long int, long int> &r) {
  return l.first < r.first;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> m;
  for (long int i = 0; i < m; i++) {
    cin >> j;
    a.push_back(make_pair(j, i));
    c.insert(j);
  }
  sort(a.begin(), a.end());
  for (long int i = 0; i < a.size(); i++) {
    if ((i + 1) < a.size()) {
      long int rag = a[i].second, bha = a[i].second;
      j = i + 1;
      while (a[i].first == a[j].first) {
        rag = max(rag, a[j].second);
        bha = min(bha, a[j].second);
        j++;
        if (j >= a.size()) break;
      }
      i = j - 1;
      b.push_back(make_pair(a[i].first, bha));
      b.push_back(make_pair(a[i].first, rag));
    } else {
      b.push_back(make_pair(a[i].first, a[i].second));
      b.push_back(make_pair(a[i].first, a[i].second));
    }
  }
  for (long int i = 1; i < n + 1; i++) {
    long int l, rag = -1, bha = -1, rag1 = -1, bha1 = -1;
    l = lower_bound(b.begin(), b.end(), pair<long int, long int>(i, 0), cmp) -
        b.begin();
    if (l >= b.size()) l--;
    if (b[l].first == i) {
      bha = b[l].second;
      rag = b[l + 1].second;
    }
    l = lower_bound(b.begin(), b.end(), pair<long int, long int>(i + 1, 0),
                    cmp) -
        b.begin();
    if (l >= b.size()) l--;
    if (b[l].first == i + 1) {
      bha1 = b[l].second;
      rag1 = b[l + 1].second;
    }
    if ((rag == -1 && bha == -1) && (rag1 == -1 && bha1 == -1))
      continue;
    else if ((rag != -1 && bha != -1) && (rag1 != -1 && bha1 != -1)) {
      if (bha < rag1) t++;
      if (bha1 < rag) t++;
    }
  }
  t += c.size();
  t = ((3 * n) - 2) - t;
  cout << t;
  return 0;
}
