#include <iostream>
#include<algorithm>
using namespace std;

int main() {
    // your code goes here
    int t;
    cin >> t;
    while( t-- ){
        long barn , cow;
        scanf("%ld%ld", &barn ,&cow );
        long stalls[barn];
        for( int i = 0; i < barn; i++ ){
            scanf("%ld", &stalls[i] );
        }
        long ans = 1;
        sort( stalls , stalls + barn );
        long int low = 1 , high = 1000000001;
        while( low <= high ){
            long int start = stalls[0] , c = 1;
            long int mid = low + ( high - low ) / 2;
            for( int i = 1; i < barn; i++ ){
                if( stalls[i] - start >= mid ){
                    c++;
                    start = stalls[i];
                }
            }
            if( c < cow ){
                high = mid - 1;
            }
            else{
                ans = mid;
                low = mid + 1;
            }
        }
        printf("%ld\n", ans );
    }
    return 0;
}