SPOJ-AGGRCOW

testCases=int(input())
for _ in range(testCases):
    N,C=[int(i) for i in input().split()]

    arr=[int(input()) for _ in range(N)]

    arr.sort()

    def ifpossible(C,mid):
        cow=1
        lastpos=0
        for i in range(1,len(arr)):
            if arr[i]-arr[lastpos]>=mid:
                cow+=1
                lastpos=i
                if cow>=C:
                    return True
        return False

    strt=0
    end=arr[-1]
    ans=-1
    mid=(end+strt)//2

    while strt<=end:
        if ifpossible(C, mid):
            ans=mid
            strt=mid+1
        else:
            end=mid-1
        mid=(end+strt)//2
    print(ans)

#**