N, P = map(int, input().split())
ABC = [list(map(int, input().split())) + [1] for _ in range(N)]
INF = 10 ** 10
dp = [0]
cnt = 1
for i in range(N):
    cnt2 = min(cnt + 3, P + 1)
    dp2 = [INF] * (cnt2)
    for k in range(cnt):
        for j, x in enumerate(ABC[i]):
            if k + j <= cnt2:
                dp2[k + j] = min(dp2[k + j], dp[k] + x)
            else:
                break
    cnt = cnt2
    dp = dp2
print(dp[P] / N)