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

signed main() {
	int N,M;
	cin >> N >> M;
	assert(N!=M);
	vector<int> A(M);
	for(int &i : A){
		cin >> i;
	}
	int ans=0;
	for(int i=0;i<M-1;i++){
		ans+=(popcount(A[i+1]-A[i])==1 ? (A[i+1]-A[i]-1)/2 : ((A[i+1]-A[i])&(A[i]-A[i+1]))-1);
	}
	cout << N-M-ans << endl;
}