#include <bits/stdc++.h>
using namespace std;
int dx[] = {-1, 1, 0, 0, 0, 0, 0, 0};
int dy[] = {0, 0, 1, -1, 0, 0, 0, 0};
const long long inf = 100000007;
const double eps = 0.000000009;
long long mod = 1000000007;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int k, n;
  cin >> k >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  long long ans = (k - 2) * 3 + 4;
  set<int> s;
  set<pair<int, int> > r;
  for (int i = 0; i < n; i++) {
    if (s.find(a[i]) == s.end()) {
      if (r.find(pair<int, int>(a[i], a[i])) == r.end()) {
        r.insert(pair<int, int>(a[i], a[i]));
      }
      s.insert(a[i]);
    }
    if (s.find(a[i] - 1) != s.end()) {
      if (a[i] - 1 == 0) continue;
      if (r.find(pair<int, int>(a[i] - 1, a[i])) == r.end()) {
        r.insert(pair<int, int>(a[i] - 1, a[i]));
      }
    }
    if (s.find(a[i] + 1) != s.end()) {
      if (a[i] + 1 == n + 1) continue;
      if (r.find(pair<int, int>(a[i] + 1, a[i])) == r.end()) {
        r.insert(pair<int, int>(a[i] + 1, a[i]));
      }
    }
  }
  cout << ans - r.size() << endl;
  return 0;
}
