#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e9;
const long long N = 1e5 + 1;
const long long mod = 1e9 + 7;
const long double eps = 1E-7;
vector<int> a[N];
int n, k, x, ans;
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n >> k;
  for (int i = 1; i <= k; i++) cin >> x, a[x].push_back(i);
  for (int i = 1; i <= n; i++) {
    if (a[i].size() == 0)
      ans += 1 + (i != 1) + (i != n);
    else {
      x = a[i][0];
      if (i != 1)
        if (upper_bound(a[i - 1].begin(), a[i - 1].end(), x) == a[i - 1].end())
          ans++;
      if (i != n)
        if (upper_bound(a[i + 1].begin(), a[i + 1].end(), x) == a[i + 1].end())
          ans++;
    }
  }
  cout << ans << endl;
}
