#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <cstring>

using namespace std;

int p,q,a[102];
int f[101][101];
bool y[101][101];
int dp(int l,int r)
{
	if (l>r) return 0;
	if (y[l][r]) return f[l][r];
	y[l][r]=true;
	f[l][r]=a[r+1]-a[l-1]-2;
	int t=2000000000;
	for (int i=l;i<=r;i++)
	{
		if (t>dp(l,i-1)+dp(i+1,r)) t=f[l][i-1]+f[i+1][r];
	}
	f[l][r]+=t;
	return f[l][r];
}

int main()
{
	freopen("prison.in","r",stdin);
	freopen("prison.out","w",stdout);
	scanf("%d%d",&p,&q);
	for (int i=1;i<=q;i++) scanf("%d",&a[i]);
	a[0]=0;a[q+1]=p+1;
	sort(a+1,a+q);
	printf("%d\n",dp(1,q));
	return 0;
}