#include <bits/stdc++.h>
using namespace std;
const int INF = (int)2e9;
const long long INFL = (long long)9e18;
const int MAXINT = ((~0) ^ (1 << 31));
const long long MAXLL = ((~0) ^ ((long long)1 << 63));
template <class T>
inline T pow2(T a) {
  return a * a;
}
template <class T>
inline bool mineq(T& a, T b) {
  return (a > b) ? (a = b, true) : false;
}
template <class T>
inline bool maxeq(T& a, T b) {
  return (a < b) ? (a = b, true) : false;
}
const int maxn = 2e5;
long long n, k, x, cnt;
pair<long long, long long> p[maxn];
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n >> k;
  for (int i = 1; i <= n; i++) p[i] = {1e10, 0};
  for (int i = 1; i <= k; i++) {
    cin >> x;
    p[x].first = min(p[x].first, (long long)i);
    p[x].second = max(p[x].second, (long long)i);
  }
  if (p[1].first > p[2].second && n != 1) {
    cnt++;
  }
  if (p[n].first > p[n - 1].second && n != 1) {
    cnt++;
  }
  if (p[1].second == 0) cnt++;
  if (p[n].second == 0 && n != 1) cnt++;
  for (int i = 2; i <= n - 1; i++) {
    if (p[i].second == 0) cnt++;
    if (p[i].first > p[i + 1].second) {
      cnt++;
    }
    if (p[i].first > p[i - 1].second) {
      cnt++;
    }
  }
  cout << cnt;
}
