import sys

def main():
    N, L = map(int, sys.stdin.readline().split())
    S = []
    sum_total = 0
    for s in sys.stdin.readline().split():
        mm, ss = map(int, s.split(':'))
        sec = mm * 60 + ss
        S.append(sec)
        sum_total += sec
    T = L * 60

    if sum_total <= T:
        print(N)
        return

    # Precompute combinations
    max_n = N - 1
    comb = [[0] * (max_n + 1) for _ in range(max_n + 1)]
    for n in range(max_n + 1):
        comb[n][0] = 1
        for k in range(1, n + 1):
            comb[n][k] = comb[n-1][k-1] + comb[n-1][k]

    expect = 0.0

    for k in range(N):
        s_list = S[:k] + S[k+1:]
        n = len(s_list)
        T_target = T
        max_j = n
        max_t = T_target - 1

        # Initialize DP
        dp = [[0] * (max_t + 1) for _ in range(max_j + 1)]
        dp[0][0] = 1

        for s in s_list:
            for j in range(max_j, 0, -1):
                for t in range(max_t, s - 1, -1):
                    if dp[j-1][t - s]:
                        dp[j][t] += dp[j-1][t - s]

        # Calculate cnt[j]
        cnt = [0] * (max_j + 1)
        for j in range(max_j + 1):
            cnt[j] = sum(dp[j][t] for t in range(T_target))

        # Calculate probability for current song k
        p = 0.0
        for i in range(1, N + 1):
            j = i - 1
            if j < 0 or j > max_j:
                continue
            c = comb[max_j][j] if j <= max_j else 0
            if c == 0:
                prob = 0.0
            else:
                prob = cnt[j] / c
            p += prob / N
        expect += p

    print("{0:.10f}".format(expect))

if __name__ == '__main__':
    main()