#include <bits/stdc++.h>
using namespace std;
int r[100005], t, n, m, ans = 0;
vector<int> id[100005];
set<int> v;
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    scanf("%d", &t);
    v.insert(t);
    id[t].push_back(i);
    r[t] = max(r[t], i);
  }
  for (auto it : v) {
    int sum = 0;
    for (int i = 0; i < id[it].size() && sum < 2; i++) {
      t = 0;
      if (id[it][i] < r[it - 1]) t++;
      if (id[it][i] < r[it + 1]) t++;
      sum = max(sum, t);
    }
    ans += sum;
  }
  printf("%d\n", n * 3 - 2 - ans - v.size());
  return 0;
}
