#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, d[100005][3] = {0}, i, j, x, tot = 0;
  cin >> n >> k;
  while (k--) {
    cin >> x;
    if (d[x][1] == 0) d[x][1] = 1;
    if (x - 1 >= 1 && d[x - 1][1] == 1) d[x - 1][2] = 1;
    if (x + 1 <= n && d[x + 1][1] == 1) d[x + 1][0] = 1;
  }
  for (i = 1; i <= n; i++)
    for (j = 0; j <= 2; j++) tot += d[i][j];
  cout << 3 * n - 2 - tot;
}
