/*对rock间的距离进行二分*/
#include<iostream>
#include <algorithm>
#define Max 50005
using namespace std;
int rock[Max];
int l,n,m,mn;
bool judge(int x)//判断可移除的rock是否大于m
{
int del=0;
for (int i=1,j=0;i<=n+1;)
{
if(rock[i]-rock[j]<=x)
{
del++;i++;
}
else
{
j=i;i++;
}
}
if(del>m)
return true;//right=mid-1;
else
return false;//left=mid+1;
}
int main()
{
while(cin>>l>>n>>m)
{
rock[0]=0;rock[n+1]=l;
for (int i=1;i<=n;i++)
{
scanf("%d",&rock[i]);
if(mn>rock[i])
mn=rock[i];
}
sort(rock+1,rock+n+1);
int right=l,left=mn;
while(left<=right)
{
int mid=(right+left)>>1;
if (judge(mid))
{
right=mid-1;
}
else
{
left=mid+1;
}
}
cout<<left<<endl;
}
return 0;
}