#include<bits/stdc++.h>
using namespace std;
int n,q,a[110],b[110]={},ans=0x3f3f3f3f;
void dfs(int x,int s)
{
	if(x==q+1)
	{
		if(s<ans)
		{
			ans=s;
		}
		return;
	}
	int s2=s;
	for(int i=1;i<=q;i++)
	{
		if(b[i]==0)
		{
			for(int j=i-1;j>=0;j--)
			{
				if(b[j]==1)
				{
					s+=a[i]-a[j]-1;
					break;
				}
			}
			for(int j=i+1;j<=q+1;j++)
			{
				if(b[j]==1)
				{
					s+=a[j]-a[i]-1;
					break;
				}
			}
			b[i]=1;
			dfs(x+1,s);
			b[i]=0;
			s=s2;
		} 
	}
}
int main()
{
	freopen("prison.in","r",stdin);
    freopen("prison.out","w",stdout);
	cin>>n>>q;
	for(int i=1;i<=q;i++)
	{
		cin>>a[i];
	}
	sort(a+1,a+q+1);
	a[0]=0;
	b[0]=1;
	a[q+1]=n+1;
	b[q+1]=1;
	dfs(1,0);
	cout<<ans;
	return 0;
}