#include<iostream>
using namespace std;
int a[50010];
int n,d,m;
bool judge(int x)
{
	int t=0,i=0,c=0;
	for(int i=1;i<=n+1;i++)
	{
	 if(a[i]-a[c]<x)
	 t++;
	 else
	 c=i;	
	}
	if(t>m)return false;
	else return true;
}
int main()
{
	cin>>d>>n>>m;
	int ans;
	for(int i=1;i<=n;i++)
	 cin>>a[i];
	a[n+1]=d; 
	 int l=0,r=d,mid;
	while(l<=r) 
	{
	 mid=(l+r)/2;
	 if(judge(mid))
	 {
	  l=mid+1;
	  ans=mid;
	 }
	 else
	 r=mid-1; 	
	}
	cout<<ans<<endl;
	return 0; 
}