/*
ID: ahshenb1
PROG: barn1
LANG: C++
*/
#include<iostream>
#include<fstream>
#include<string>
using namespace std;

void qsort(int a[],int i,int j)
{
   if(i>=j) return;
   int l=i,r=j,mid=a[(i+j)/2];

   while(l<=r){
        while(a[l]<mid) ++l;
        while(a[r]>mid) --r;
        if(l<=r){
            int t = a[l];
            a[l] = a[r];
            a[r] = t;
            ++l;
            --r;
        }
   }

   qsort(a,i,r);
   qsort(a,l,j);
}
int a[10000],b[10000];
int main()
{
    ifstream fin("barn1.in");
    ofstream fout("barn1.out");
    int select,all,n;

    fin>>select>>all>>n;
    for(int i=0;i<n;++i) fin>>a[i];
    qsort(a,0,n-1);
    b[0]=0;
    for(int i=1;i<n;++i)
        b[i]=a[i]-a[i-1];
    qsort(b,0,n-1);
//    for(int i =0;i<n;++i)
//        cout<<b[i]<<endl;
    int t=0;
    for(int i=0;i<select-1;++i)
        t+=b[n-1-i];
    //cout<<t<<endl;
    int result=a[n-1]-a[0]-t+select;
    fout<<result<<endl;

    return 0;
}