def group(N,K,N2):
    if N % K == 0:
        return [N2[i:i+(N/K)] for i in range(0,len(N2),(N/K))]
    else:
        return [N2[i:i+(N%K)] for i in range(0,len(N2),(N%K))]

N = int(input())
K = int(input())
N2 = [int(input()) for i in range(N)]
r = [sum(i)/len(i) for i in group(N,K,N2)]
r.sort()
print(r[0] - r[-1])