#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <string>
#include <cstdio>
#include <queue>
#include <ctime>
using namespace std;
int i,j,k,l,m,n,a[55000],r,mid,ans,s;
int check()
{
  	int i,j,k=0;
  	for (i=0,j=1;j<=n;j++)
  	  {
  	  	if (a[j]-a[i]<mid)
  	  	  {
  	  	  	k++;continue;
		  }
		i=j;
	  }
	if (s-a[i]<mid||k>m) return 0;
	return 1;
  }
int main()
{
  	scanf("%d%d%d",&s,&n,&m);
  	for (i=1;i<=n;i++) scanf("%d",&a[i]);
  	sort(a+1,a+n+1);
  	for (l=0,r=s;l<=r;)
  	  {
  	  	mid=(l+r)/2;
  	  	if (check()) l=mid+1,ans=mid;else r=mid-1;
      }
    printf("%d",ans);
    return 0;
}