#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long i, n, j, k, x[100003], a[4][100003], ans;
  cin >> n >> k;
  for (i = 0; i <= n + 1; i++) {
    a[1][i] = 1;
    a[2][i] = 1;
    a[3][i] = 1;
  }
  a[1][1] = 0;
  a[3][n] = 0;
  for (i = 1; i <= k; i++) {
    cin >> x[i];
  }
  ans = 0;
  for (j = 1; j <= k; j++) {
    i = x[j];
    if (a[2][i] == 1) {
      a[2][i] = 0;
    }
    if (a[2][i - 1] == 0) {
      a[3][i - 1] = 0;
    }
    if (a[2][i + 1] == 0) {
      a[1][i + 1] = 0;
    }
  }
  for (i = 1; i <= n; i++) {
    if (a[1][i]) ans++;
    if (a[2][i]) ans++;
    if (a[3][i]) ans++;
  }
  cout << ans;
}
