#![allow(non_snake_case)]
#![allow(unused_imports)]
// use itertools::Itertools;
// use proconio::input;
// use proconio::marker::*;
use std::cmp::Reverse;
use std::collections::*;

fn main() {
    input! {
        n:usize,
        k:usize,
        ab:[(usize,usize);n],
    }

    let judge = |x: usize| {
        let mut cnt = 0;
        for &(a, b) in ab.iter() {
            cnt += x.min(b).saturating_sub(a - 1);
        }

        k <= cnt
    };

    let ok = ab.iter().map(|(_, b)| *b).max().unwrap();
    let ans = nibutan(ok, 0, judge);
    println!("{}", ans);
}

use nibutan::*;
mod nibutan {
    pub fn nibutan<F: Fn(usize) -> bool>(mut ok: usize, mut ng: usize, judge: F) -> usize {
        while ok.abs_diff(ng) > 1 {
            let mid = (ok + ng) / 2;
            if judge(mid) {
                ok = mid;
            } else {
                ng = mid;
            }
        }

        ok
    }
}

mod input {
    #[macro_export]
    macro_rules! input {
        (source = $s:expr, $($r:tt)*) => {
            let mut iter = $s.split_whitespace();
            input_inner!{iter, $($r)*}
        };
        ($($r:tt)*) => {
            let s = {
                use std::io::Read;
                let mut s = String::new();
                std::io::stdin().read_to_string(&mut s).unwrap();
                s
            };
            let mut iter = s.split_whitespace();
            input_inner!{iter, $($r)*}
        };
    }

    #[macro_export]
    macro_rules! input_inner {
        ($iter:expr) => {};
        ($iter:expr, ) => {};

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

    #[macro_export]
    macro_rules! read_value {
        ($iter:expr, ( $($t:tt),* )) => {
            ( $(read_value!($iter, $t)),* )
        };

        ($iter:expr, [ $t:tt ; $len:expr ]) => {
            (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
        };

        ($iter:expr, chars) => {
            read_value!($iter, String).chars().collect::<Vec<char>>()
        };

        ($iter:expr, usize1) => {
            read_value!($iter, usize) - 1
        };

        ($iter:expr, $t:ty) => {
            $iter.next().unwrap().parse::<$t>().expect("Parse error")
        };
    }
}