#include <bits/stdc++.h>
using namespace std;
int main(){
  long long N;
  int M;
  cin >> N >> M;
  vector<long long> A(M);
  for (int i = 0; i < M; i++){
    cin >> A[i];
  }
  long long ans = N - M;
  for (int i = 0; i < M - 1; i++){
    if (A[i + 1] - A[i] > 1){
      long long d = A[i + 1] - A[i];
      if (d % 2 == 0 & d >= 4){
        if (d % 4 == 2){
          ans--;
        } else if (d == 4){
          ans--;
        } else {
          ans -= (long long) 1 << (__builtin_ctzll(d) - 1);
          ans++;
        }
      }
    }
  }
  cout << ans << endl;
}