#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 (__builtin_popcountll(d) == 1){
        ans -= d / 2 - 1;
      } else {
        ans -= ((long long) 1 << __builtin_ctzll(d)) - 1;
      }
    }
  }
  cout << ans << endl;
}