#include <bits/stdc++.h>
using namespace std;
int a[100010], v[100010][3];
int main() {
  int n, k, ans = 0;
  cin >> n >> k;
  for (int i = 0; i < k; i++) cin >> a[i];
  for (int i = 0; i <= n + 1; i++) v[i][0] = v[i][1] = v[i][2] = 1;
  for (int i = k - 1; i >= 0; i--) {
    v[a[i]][1] = 0;
    if (v[a[i] + 1][1] == 0) v[a[i]][2] = 0;
    if (v[a[i] - 1][1] == 0) v[a[i]][0] = 0;
  }
  for (int i = 2; i < n; i++) {
    ans += v[i][0] + v[i][1] + v[i][2];
  }
  ans += v[1][1];
  if (n > 1) ans += v[1][2] + v[n][1] + v[n][0];
  cout << ans << "\n";
  return 0;
}
