#![allow(non_snake_case)]

type Item = (u64, u64);

fn compare(item1: &Item, item2: &Item) -> bool {
    item1.0 >= item2.0 || item1.1 >= item2.1
}

fn enumerate_items(items: &[Item], capacity: u64) -> Vec<Item> {

    let mut a0: Vec<Item> = vec![(0, 0)];
    for &(weight, value) in items {
        let N0: usize = a0.len();
        let mut a1: Vec<Item> = Vec::new();
        for &(w0, v0) in &a0 {
            let w1: u64 = w0 + weight;
            if w1 > capacity {
                break;
            }
            let v1: u64 = v0 + value;
            a1.push((w1, v1));
        }
        let N1: usize = a1.len();

        let mut b: Vec<Item> = vec![(0, 0)];
        let mut i0: usize = 1;
        let mut i1: usize = 0;
        loop {
            if i0 == N0 {
                while i1 < N1 {
                    b.push(a1[i1]);
                    i1 += 1;
                }
                break;
            } else if i1 == N1 {
                while i0 < N0 {
                    b.push(a0[i0]);
                    i0 += 1;
                }
                break;
            } else {
                if a0[i0].0 < a1[i1].0 {
                    b.push(a0[i0]);
                    i0 += 1;
                    let b_last = b.last().unwrap();
                    while i1 < N1 && compare(b_last, &a1[i1]) {
                        i1 += 1;
                    }
                } else if a0[i0].0 > a1[i1].0 {
                    b.push(a1[i1]);
                    i1 += 1;
                    let b_last = b.last().unwrap();
                    while i0 < N0 && compare(b_last, &a0[i0]) {
                        i0 += 1;
                    }
                } else {
                    if a0[i0].1 >= a1[i1].1 {
                        b.push(a0[i0]);
                        i0 += 1;
                        i1 += 1;
                        let b_last = b.last().unwrap();
                        while i1 < N1 && compare(b_last, &a1[i1]) {
                            i1 += 1;
                        }
                    } else {
                        b.push(a1[i1]);
                        i1 += 1;
                        i0 += 1;
                        let b_last = b.last().unwrap();
                        while i0 < N0 && compare(b_last, &a0[i0]) {
                            i0 += 1;
                        }
                    }
                }
            }
        }
        a0 = b;
    }
    a0
}

fn knapsack_01_small_items(items: &[Item], capacity: u64) -> u64 {

    let N: usize = items.len();
    let N0: usize = N / 2;

    let a0 = enumerate_items(&items[..N0], capacity);
    let a1 = enumerate_items(&items[N0..], capacity);

    let mut result: u64 = 0;
    let mut j: usize = a1.len() - 1;
    'outer: for &(w0, v0) in &a0 {
        let res: u64 = capacity - w0;
        while a1[j].0 > res {
            if j == 0 {
                break 'outer;
            }
            j -= 1;
        }
        result = std::cmp::max(result, v0 + a1[j].1);
    }
    result
}

fn main() {
    let stdin = std::io::stdin();
    let mut stdin = my::io::InputReader::new(stdin.lock());

    let N: usize = stdin.read();
    let W: u64 = stdin.read();
    let mut items = Vec::<(u64, u64)>::new();
    for _ in 0..N {
        let v: u64 = stdin.read();
        let w: u64 = stdin.read();
        items.push((w, v));
    }
    println!("{}", knapsack_01_small_items(&items, W));
}

#[allow(unused_must_use)]
mod my {
    pub mod io {
        use std::io::{BufRead, ErrorKind, Result};
        use std::str::{FromStr, from_utf8};
        use std::fmt::Debug;

        pub fn read_word<R: BufRead>(r: &mut R, buf: &mut Vec<u8>)
                                              -> Result<usize> {
            let mut read = 0;
            loop {
                let (done, used) = {
                    let available = match r.fill_buf() {
                        Ok(n) => n,
                        Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                        Err(e) => return Err(e)
                    };
                    match available.iter().position(|&x| x == b'\n' || x == b' ') {
                        Some(i) => {
                            buf.extend_from_slice(&available[..i]);
                            (true, i + 1)
                        }
                        None => {
                            buf.extend_from_slice(available);
                            (false, available.len())
                        }
                    }
                };
                r.consume(used);
                read += used;
                if done || used == 0 {
                    return Ok(read);
                }
            }
        }

        pub struct InputReader<R: BufRead> {
            reader: R,
            buf: Vec<u8>,
        }

        impl<R: BufRead> InputReader<R> {
            pub fn new(reader: R) -> InputReader<R> {
                InputReader {
                    reader: reader,
                    buf: Vec::<u8>::new(),
                }
            }

            pub fn read_word(&mut self) {
                read_word(&mut self.reader, &mut self.buf);
            }
            
            pub fn read<T>(&mut self) -> T
                where T: FromStr,
                      T::Err: Debug
            {
                self.buf.clear();
                self.read_word();
                from_utf8(&self.buf).unwrap().trim().parse().unwrap()
            }
        }
    }
}



Measured memory_bytes: 5028