n, k = [int(i) for i in input().split()]
dots = [int(i)-1 for i in input().split()]

first = dots[0]
last = dots[len(dots) - 1]

max_l = (last - first) // k + (1 if (last - first) % k != 0 else 0)

i = max_l

while True:
    cur_pos = first
    l = 0
    f = False
    while True:
        cur_pos += i
        l += 1
        if cur_pos in dots:
            if cur_pos == last:
                if l<=k:
                    f = True
                    break
        else:
            break
    if f:
        break
    i += 1
print(i)