3_O

def k_th_differet(arr, K):
    counter = [0] * (max(arr) + 1)
    for i in range(len(arr) - 1):
        for j in range(i + 1, len(arr)):
            diff = abs(arr[j] - arr[i])
            counter[diff] += 1
    for i in range(len(counter)):
        K -= counter[i]
        if K <= 0:
            return i


n = int(input())
areas = list(map(int, input().split()))
k = int(input())
print(k_th_differet(areas, k))