n, k = [int(x.strip()) for x in input().strip().split()]
evals = []
for _ in range(n):
    evals.append( int(input().strip()) )

evals.sort()

sx = 0.0
sxx = 0.0
for i in range(k):
    x = evals[i]
    sx += x
    sxx += x*x

badness = sxx - sx*sx/k

for i in range(k,n):
    oldx = evals[i-k]
    newx = evals[i]
    sx += newx-oldx
    sxx += newx*newx - oldx*oldx
    new_badness = sxx - sx*sx/k
    if new_badness<badness:
        badness = new_badness
        
print( badness )