#include <bits/stdc++.h>
using namespace std;
int n, m, k, i;
int main() {
  cin >> n >> k;
  unordered_map<int, unordered_set<int>> m;
  unordered_set<int> s;
  int a[k];
  cin >> a[0];
  s.insert(a[0]);
  for (int i = 1; i < k; i++) {
    cin >> a[i];
    if (s.find(a[i] - 1) != s.end()) {
      m[a[i] - 1].insert(a[i]);
    }
    if (s.find(a[i] + 1) != s.end()) {
      m[a[i] + 1].insert(a[i]);
    }
    s.insert(a[i]);
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    if (s.find(i) == s.end() && i != 1 && i != n)
      ans += 3;
    else if (s.find(i) == s.end() && (i == 1 || i == n))
      ans += 2;
    else if (s.find(i) != s.end() && i != 1 && i != n) {
      ans += 2 - m[i].size();
    } else if (s.find(i) != s.end() && (i == 1 || i == n))
      ans += 1 - m[i].size();
  }
  if (n == 1) ans = 0;
  cout << ans;
}
