fn main() {
    let (n, m): (usize, i64) = io::input();
    let ab = (0..n)
        .map(|_| io::input::<(i64, i64)>())
        .collect::<Vec<_>>();

    let mut dp = [0; 3];
    {
        let (a, b) = ab[0];
        dp[1] = (a * m).max(a * (m - 1) + b).max(b);
    }
    let mut ans = dp[1];
    for &(a, b) in &ab[1..] {
        dp[0] = dp[0].max(dp[1]);
        dp[0] += a.max(b);
        dp[1] += (a * m).max(a * (m - 1) + b).max(b);
        ans = ans.max(dp[0]).max(dp[1]);
    }
    println!("{ans}");
}

// io {{{
// https://ngtkana.github.io/ac-adapter-rs/io/index.html

#[allow(dead_code)]
mod io {
    use std::cell::Cell;
    use std::io::stdin;
    use std::io::BufRead;
    use std::io::BufReader;
    use std::io::Lines;
    use std::io::Stdin;
    use std::sync::Mutex;
    use std::sync::Once;
    pub fn input<T: ParseLine>() -> T {
        ParseLine::parse_line(&line())
    }
    pub trait ParseLine {
        fn parse_line(s: &str) -> Self;
    }
    macro_rules! impl_parse_line {
        ($($t:ty),*) => {
            $(impl ParseLine for $t {
                fn parse_line(s: &str) -> Self {
                    s.parse().unwrap()
                }
            })*
        };
    }
    impl_parse_line!(u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize, String, char);
    macro_rules! impl_parse_line_tuple {
        ($($t:ident),*) => {
            impl<$($t: ParseLine),*> ParseLine for ($($t,)*) {
                fn parse_line(s: &str) -> Self {
                    let mut s = s.split_whitespace();
                    ($($t::parse_line(s.next().unwrap()),)*)
                }
            }
        };
    }
    impl_parse_line_tuple!(T0, T1);
    impl_parse_line_tuple!(T0, T1, T2);
    impl_parse_line_tuple!(T0, T1, T2, T3);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4, T5);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4, T5, T6);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4, T5, T6, T7);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4, T5, T6, T7, T8);
    impl_parse_line_tuple!(T0, T1, T2, T3, T4, T5, T6, T7, T8, T9);
    impl<T: ParseLine> ParseLine for Vec<T> {
        fn parse_line(s: &str) -> Self {
            s.split_whitespace().map(T::parse_line).collect()
        }
    }
    static ONCE: Once = Once::new();
    type Server = Mutex<Lines<BufReader<Stdin>>>;
    struct Lazy(Cell<Option<Server>>);
    unsafe impl Sync for Lazy {}
    fn line() -> String {
        static SYNCER: Lazy = Lazy(Cell::new(None));
        ONCE.call_once(|| {
            SYNCER
                .0
                .set(Some(Mutex::new(BufReader::new(stdin()).lines())));
        });
        unsafe {
            (*SYNCER.0.as_ptr())
                .as_ref()
                .unwrap()
                .lock()
                .unwrap()
                .next()
                .unwrap()
                .unwrap()
        }
    }
}
// }}}