#include<bits/stdc++.h>
using namespace std;
int main(){
	long long N,M;
	cin>>N>>M;
	map<long long,long long> mp;
	function<long long(long long)> f=[&](long long L){
		if(mp.count(L))return mp[L];
		else if(L<3 or L%2==0)return L;
		else return mp[L]=f(L/2)+f(L-L/2-1);
	};
	long long ans=0;
	vector<long long> A(M);
	for(auto &&a:A)cin>>a;
	for(int i=0;i<M-1;i++){
		ans+=f(A[i+1]-A[i]-1);
	}
	cout<<ans<<endl;
}