#include<bits/stdc++.h>
using namespace std;

   
bool ok(vector<int>&arr,int m,int x){
  int cnt=1;
  int i=0;
  int n=arr.size();
  while(cnt<m){
    int next=arr[i]+x;
    int low2=i+1;
    int high2=n-1;
    int ans2=-1;
    while(low2<=high2){
      int mid2=(low2+high2)/2;
      if(arr[mid2]>=next){
        ans=mid2;
        high2=mid2-1;
      }else{
        low2=mid2+1;
      }
    }
    if(ans==-1)return false;
    i=ans;
    cnt++;
  }

  if(cnt==m)return true;
  else return false;
}
int findMaximum(vector<int>arr,int m){
  int n=arr.size();
  int low=0;
  int high=1e9;
  int ans=-1;
  while(low<=high){
    int mid=(low+high)/2;
    if(ok(arr,m,mid)){
      ans=mid;
      low=mid+1;
    }else{
      high=mid-1;
    }
  }

  return ans;
}
int main()
{
 
  int n;
  cin>>n;
  vector<int>v(n);
  cin>>v;
  int m;
  cin>>m;
  cout<<findMaximum(v,m)<<endl;
}