import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np

N, L = map(int, readline().split())
L *= 60

comb = np.zeros((60, 60), np.int64)
comb[0, 0] = 1
for n in range(1, 60):
    comb[n] += comb[n - 1]
    comb[n, 1:] += comb[n - 1, :-1]


def convert(s):
    minute, second = map(int, s.split(b':'))
    return 60 * minute + second


S = tuple(map(convert, read().split()))
full = sum(S)
if L >= full:
    print(N)
    exit()

dp = np.zeros((N + 1, L + 1), np.int64)
dp[0, 0] = 1


def add(dp, x):
    if x > L:
        return
    for n in range(N, 0, -1):
        dp[n, x:] += dp[n - 1, :-x]


def remove(dp, x):
    if x > L:
        return
    for n in range(1, N + 1):
        dp[n, x:] -= dp[n - 1, :-x]


for x in S:
    add(dp, x)

expected = 0
for x in S:
    remove(dp, x)
    p = 0
    for n in range(N):
        p += dp[n, :L].sum() / comb[N - 1, n]
    p /= N
    expected += p
    add(dp, x)

print(expected)