n, m = map(int, input().split())
c = list(map(int, input().split()))

def move_prob(pos, P_list):
	if pos == n-1:   # 最後の1回はズル
		return sum(P_list[-1-m:])
	prob = sum(P_list[max(0, pos-m):min(n-m-1, pos)]) / m
	return prob

OLD_P = [1] + [0]*(n-1)
EXP_GOLD = 0
for _ in range(n-m):
	# サイコロを振って確率の更新
	NEW_P = [move_prob(pos, OLD_P) for pos in range(n)]
	# 部分期待値の計算
	subgold = sum([i*j for i, j in zip(c, NEW_P[1:-1])])
	EXP_GOLD += subgold
	OLD_P = NEW_P

print(EXP_GOLD)