#include <bits/stdc++.h>
using namespace std;
const int mn = 100010;
int a[mn];
int dt[mn][3];
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = 0; i < k; ++i) scanf("%d", a + i);
  for (int i = 1; i < n + 1; ++i) {
    dt[i][0] = dt[i][1] = dt[i][2] = 0;
  }
  unordered_set<int> s;
  for (int i = k - 1; i >= 0; --i) {
    if (dt[a[i]][0] == 0) dt[a[i]][0] = 1;
    if (a[i] != n && s.find(a[i] + 1) != s.end() && dt[a[i]][1] == 0)
      dt[a[i]][1] = 1;
    if (a[i] != 1 && s.find(a[i] - 1) != s.end() && dt[a[i]][2] == 0)
      dt[a[i]][2] = 1;
    s.insert(a[i]);
  }
  int ans = 3 * n - 2;
  for (int i = 1; i <= n; ++i) {
    ans -= dt[i][0] + dt[i][1] + dt[i][2];
  }
  cout << ans << endl;
  return 0;
}
