def test_distance(cows: int, locations: list[int], distance: int) -> bool:
    placed_cows = 1
    last = locations[0]
    for location in locations[1:]:
        if location - last >= distance:
            last = location
            placed_cows += 1

        if placed_cows >= cows:
            return True

    return False

def main():
    test_cases: int = int(input())

    while test_cases != 0:

        stalls, cows = [int(n) for n in input().split()]

        locations = []
        i = 0
        while i < stalls:

            locations.append(int(input()))

            i += 1

        locations.sort()

        l = locations[1] - locations[0]
        r = max(locations) - min(locations)
        m = (l + r) // 2

        while True:
            old_m = m

            # Current split worked, so try more
            if test_distance(cows, locations, m):
                l = m + 1
            else:
                r = m - 1

            m = (l + r) // 2

            if m == old_m:
                print(m)
                break


        test_cases -= 1

if __name__ == '__main__':
    main()