#[allow(unused_imports)]
use std::cmp::*;
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes.by_ref().map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr,) => {};
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => { ($(read_value!($next, $t)),*) };
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };
    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };
    ($next:expr, usize1) => (read_value!($next, usize) - 1);
    ($next:expr, [ $t:tt ]) => {{
        let len = read_value!($next, usize);
        read_value!($next, [$t; len])
    }};
    ($next:expr, $t:ty) => ($next().parse::<$t>().expect("Parse error"));
}

trait Change { fn chmax(&mut self, x: Self); fn chmin(&mut self, x: Self); }
impl<T: PartialOrd> Change for T {
    fn chmax(&mut self, x: T) { if *self < x { *self = x; } }
    fn chmin(&mut self, x: T) { if *self > x { *self = x; } }
}

fn parse(s: &str) -> usize {
    let s: Vec<_> = s.split(":").collect();
    assert_eq!(s.len(), 2);
    let m: usize = s[0].parse().unwrap();
    let s: usize = s[1].parse().unwrap();
    60 * m + s
}

fn calc(l: usize, rest: &[usize]) -> f64 {
    let n = rest.len() + 1;
    let mut dp = vec![vec![0.0; l]; n];
    dp[0][0] = 1.0;
    for i in 0..n - 1 {
        let mut ep = dp.clone();
        for u in 0..n {
            for j in 0..l {
                let val = dp[u][j];
                if u + 1 < n && j + rest[i] < l {
                    ep[u + 1][j + rest[i]] += val;
                }
            }
        }
        dp = ep;
    }
    let mut comb = vec![vec![0.0; n]; n];
    comb[0][0] = 1.0;
    for i in 1..n {
        for j in 0..n {
            comb[i][j] = comb[i - 1][j];
            if j > 0 {
                comb[i][j] += comb[i - 1][j - 1];
            }
        }
    }
    let mut ans = 0.0;
    for i in 0..n {
        for j in 0..l {
            // (* i! * (n - i - 1)! / n!) = (/ n / comb[n - 1][i])
            ans += dp[i][j] / comb[n - 1][i];
        }
    }
    ans / n as f64
}

fn main() {
    input! {
        n: usize, l: usize,
        s: [String; n],
    }
    let l = 60 * l;
    let s: Vec<_> = s.iter().map(|x| parse(x)).collect();
    let mut ans = 0.0;
    for i in 0..n {
        let mut b = s.clone();
        b.remove(i);
        ans += calc(l, &b);
    }
    println!("{}", ans);
}