fuck_karl

#include <iostream>
#include <queue>
#include <map>
#define int long long
using namespace std;
struct node
{
	int l,r;
	int min()
	{
		if(l<r)return l;
		else return r;
	}
};
bool operator >(node a,node b)
{
	if(a.r-a.l!=b.r-b.l)return a.r-a.l>b.r-b.l;
	return a.l<b.l;
}
bool operator <(node a,node b)
{
	return !(a>b);
}
int n,m,a[1000005],ans;
map<int,node>pk;
map<int,int>cr;
priority_queue<node>q;
signed main()
{
	freopen("parking.in","r",stdin);
	freopen("parking.out","w",stdout);
	cin >> n >> m;
	for(int i = 1;i<=m;i++)scanf("%lld",&a[i]);
	q.push({0ll,n+1});
	for(int i = 1;i<=m;i++)
	{
		int l=q.top().l;
		int r=q.top().r;
		int mid=(l+r)>>1;
		q.pop();
		pk[i]={mid-l,r-mid};
		if(l==0)pk[i].l=1e18;
		if(r==n+1)pk[i].r=1e18;
		cr[mid]=i;
		pk[cr[l]].r=mid-l;
		pk[cr[r]].l=r-mid;
		if(r-mid>1)q.push({mid,r});
		if(mid-l>1)q.push({l,mid});
	}
	for(int i = 1;i<=m;i++)
	{
		ans+=max(0ll,a[i]-pk[i].min()+1);
	}
	cout << ans;
	return 0;
}