#include<bits/stdc++.h>
using namespace std;
#define int uint64_t

int f(int N){
	if(__builtin_popcount(N+1)==1) return N/2;
	int ans=(N+1)&(-N-1);
    return ans-1;
}

signed main() {
	int N,M;
	cin >> N >> M;
	vector<int> A(M);
	for(int &i : A){
		cin >> i;
	}
	int ans=0;
	for(int i=0;i<M-1;i++){
		ans+=f(A[i+1]-A[i]-1);
	}
	cout << N-M-ans << endl;
}