#include <iostream>

#include <cstdio>

#include <cmath>

#include <algorithm>

#include <vector>

#include <iomanip>

#include <cstring>

#include <map>

#include <queue>

#include <set>

#include <cassert>

#include <stack>

#define mkp make_pair

using namespace std;

const double EPS=1e-;

typedef long long lon;

const lon SZ=,INF=0x7FFFFFFF;

lon arr[SZ],cut[SZ];

struct nd{

    int a,b;

    nd(int x,int y):a(x),b(y){}

    bool operator<(const nd &rbs)const

    {

        return a<rbs.a;

    }

};

int main()

{

    std::ios::sync_with_stdio();

    //freopen("d:\\1.txt","r",stdin);

    lon casenum;

    //cin>>casenum;

    //for(lon time=1;time<=casenum;++time)

    {

        lon k,tot,n;

        cin>>k>>tot>>n;

        priority_queue<nd> pq;

        for(int i=;i<=n;++i)

        {

            cin>>arr[i];

        }

        sort(arr+,arr++n);

        for(int i=;i<=n;++i)

        {

            if(i!=&&arr[i]-arr[i-]>)

            {

                pq.push(nd(arr[i]-arr[i-],i));

            }

        }

        for(;k!=;)

        {

            if(pq.empty())--k;

            else

            {

                nd pos=pq.top();

                pq.pop();

                //cout<<"pos.a: "<<pos.a<<endl;

                cut[pos.b]=;

                --k;

            }

        }

        int bg=;

        int res=;

        for(int i=;i<=n;++i)

        {

            if(cut[i])

            {

                //cout<<"i: "<<i<<endl;

                res+=arr[i-]-arr[bg]+;

                //cout<<(arr[i-1]-arr[bg]+1)<<endl;

                bg=i;

            }

        }

        res+=arr[n]-arr[bg]+;

        cout<<res<<endl;

    }

    return ;

}