#include <iostream>
#include <vector>
using namespace std;
int N,M,d,now,diff,memo,ans;
#define INF 1e9;
vector<int> D;
int main(){
	cin>>N>>M;
	for(int i=0;i<M;i++){
		cin>>d;
		D.push_back(d);
	}
	now=0;
	if(now==0)N--;
	for(int k=0;k<N;k++){
        diff=INF;
		for(int i=0;i<M;i++){
			if(abs(D[i]-now)<diff&&abs(D[i]-now)!=0)
			{
				diff = abs(D[i] - now);
				memo = D[i];
			}
		}
        //cout<<now<<endl;
		ans+=diff;
		now = memo;
	}
    cout<<ans<<endl;
}