#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("2015stone.in");
ofstream fout("2015stone.out");
int stone[100000],cha[100000];
bool cmp(int x,int y)
{
	return x<y;
}

int main()
{
	int l,m,n,i;
	fin>>l>>n>>m;
	for(i=1;i<=n;i++)
		fin>>stone[i];
	cha[0]=stone[i];
	for(i=1;i<n;i++)
		cha[i]=stone[i+1]-stone[i];
	cha[n]=l-stone[n];
	sort(cha,cha+n,cmp);
	if(m!=n)
	fout<<cha[1+m]<<endl;
	else
		fout<<l<<endl;
	fin.close();
	fout.close();
	return 0;
}