import java.util.*;
import java.lang.*;
class Ideone
{ static boolean Cows(int arr[],int dist,int cows){
int Cnt=1,last=arr[0];
for(int i=1;i<arr.length;i++){
if(arr[i]-last>=dist){
Cnt++;
last=arr[i];
}
if(Cnt>=cows) return true;
}
return false;
}
static int BS(int arr[],int C,int N){
int low=1,high=arr[N-1]-arr[0];
while(low<=high){
int mid=(low+high)/2;
if(Cows(arr,mid,C)) low=mid+1;
else high=mid-1;
}
return high;
}
{
Scanner sc
=new Scanner
(System.
in); int T=sc.nextInt();
while(T>0){
int N=sc.nextInt();
int C=sc.nextInt();
int arr[]=new int[N];
for(int i=0;i<N;i++){
arr[i]=sc.nextInt();
}
System.
out.
println(BS
(arr,C,N
)); T--;
}
}
}
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