# @author godmar

# my guess here would be that the k numbers are consecutive when sorted
n, k = map(int, input().split())
x = [int(input()) for i in range(n)]
x.sort()

S_X = sum(x[:k])
S_X2 = sum(_x*_x for _x in x[:k])
badness = S_X2 - S_X * S_X/k
for i in range(k, n):
    S_X2 += -x[i-k] * x[i-k] + x[i] * x[i]
    S_X += -x[i-k] + x[i]
    badness = min(badness, S_X2 - S_X * S_X/k)
   
print (badness)