/******************
*  Author: BD747  *
******************/
/*


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*/
#include<algorithm>
#include<cstdio>
#include<iostream>
#include<cstring>
#include<cmath>
#include<map>
#include<queue>
#include<set>
#include<sstream>
#include<stack>
#include<string>
#include<vector>
#include<bits/stdc++.h>
using namespace std;

int n,m;
int x[100005];
int f[100005];
int ans;

int main(){

    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin>>n>>m;
    for(int i=1;i<=m;i++){
        cin>>x[i];
    }
    sort(x+1,x+1+m);
    for(int i=1;i<=m;i++){
        f[i]=x[i+1]-x[i];
    }
    sort(f+1,f+m);
    for(int i=1;i<=m-n;i++){
        ans+=f[i];
    }
    cout<<ans<<endl;

    return 0;
}

Measured memory_bytes: 1024