#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;
}
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