#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

ll M,N;
ll A[202020];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>M;
	map<ll,ll> X;
	A[0]=0,A[M+1]=N+1;
	FOR(i,M) {
		cin>>A[i+1];
		X[A[i+1]-A[i]-1]++;
	}
	X[A[M+1]-A[M]-1]++;
	ll ret=0;
	while(1) {
		X.erase(0);
		if(X.empty()) break;
		ll a=X.rbegin()->first;
		ll b=X[a];
		X.erase(a);
		if(a%2==0||a==1) {
			ret+=a*b;
		}
		else {
			X[a/2]+=2*b;
		}
	}
	cout<<ret<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}