#include <bits/stdc++.h>
using namespace std;
const int N = 1e3+10;
int n,m;
long long a[N],f[N][N];
int main(){
    freopen("prison.in","r",stdin);
    freopen("prison.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(int i = 1;i <= n;i++)scanf("%lld",&a[i]);
    sort(a+1,a+1+n);
    a[n+1] = m+1;
    memset(f,0x3f,sizeof(f));
    for(int i = 1;i <= n;i++)f[i][i] = a[i+1] - a[i-1] - 2;//初始化 
    for(int l = 2;l <= n;l++){
        for(int i = 1;i+l-1 <= n;i++){
            int j = i+l-1;
            f[i][j] = min(f[i][j],f[i+1][j]+a[j+1]-a[i-1]-2);
            for(int k = i+1;k < j;k++)f[i][j] = min(f[i][j],f[i][k-1]+f[k+1][j]+a[j+1]-a[i-1]-2);
            f[i][j] = min(f[i][j],f[i][j-1]+a[j+1]-a[i-1]-2);
        }
    }
    printf("%lld\n",f[1][n]);
    
    return 0;
    
}