Problem:
Score : 300 points

Problem Statement

Hearing that energy drinks increase rating in those sites, Takahashi decides to buy up M cans of energy drinks.

There are N stores that sell energy drinks. In the i-th store, he can buy at most B_i cans of energy drinks for A_i yen (the currency of Japan) each.

What is the minimum amount of money with which he can buy M cans of energy drinks?

It is guaranteed that, in the given inputs, a sufficient amount of money can always buy M cans of energy drinks.

Constraints

All values in input are integers.

1 \leq N, M \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^5

B_1 + ... + B_N \geq M

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the minimum amount of money with which Takahashi can buy M cans of energy drinks.

Sample Input 1

2 5
4 9
2 4

Sample Output 1

12

With 12 yen, we can buy one drink at the first store and four drinks at the second store, for the total of five drinks. However, we cannot buy 5 drinks with 11 yen or less.

Sample Input 2

4 30
6 18
2 5
3 10
7 9

Sample Output 2

130

Sample Input 3

1 100000
1000000000 100000

Sample Output 3

100000000000000

The output may not fit into a 32-bit integer type.

Solution in Rust (1.15.1) (judge verdict: Accepted)

use contest::io::Scanner;

fn main() {
    let mut sc = Scanner::new(std::io::stdin());
    let n = sc.next::<usize>();
    let mut m = sc.next::<i64>();
    let mut v: Vec<(i64, i64)> = vec![];

    for _ in 0..n {
        v.push((sc.next(), sc.next()));
    }
    v.sort();
    let mut res = 0;
    for &(a, b) in &v {
        if m <= 0 {
            break;
        }
        for _ in 0..b {
            if m <= 0 {
                break;
            }
            m -= 1;
            res += a;
        }
    }
    println!("{}", res);
}
pub mod contest {
    pub mod io {
        /// Scanner
        ///
        /// # Example
        /// ```
        /// use contest::io::Scanner;
        /// let mut sc = Scanner::new("1 2 \n\n \r\t \n 3.5".as_bytes());
        /// assert_eq!("1".to_string(), sc.next::<String>());
        /// assert_eq!(2, sc.next());
        /// assert_eq!(3.5, sc.next());
        ///
        /// // To create a scanner from stdin:
        /// Scanner::new(std::io::stdin());
        /// ```
        use std;
        use std::io;
        use std::io::BufRead;

        pub struct Scanner<R: io::Read> {
            br: io::BufReader<R>,
            // Read tokens are stored in reversed order per line.
            buf: Vec<String>,
        }

        pub fn new<R: io::Read>(r: R) -> Scanner<R> {
            Scanner::new(r)
        }

        impl<R: io::Read> Scanner<R> {
            #[inline]
            pub fn new(r: R) -> Scanner<R> {
                Scanner {
                    br: io::BufReader::new(r),
                    buf: vec![],
                }
            }
            #[inline]
            pub fn next<T>(&mut self) -> T
            where
                T: std::str::FromStr,
                T::Err: std::fmt::Debug,
            {
                self.next_string()
                    .map(|s| s.parse::<T>().expect("Parse failed: "))
                    .expect("Unexpected EOF")
            }
            #[inline]
            pub fn next2<T1, T2>(&mut self) -> (T1, T2)
            where
                T1: std::str::FromStr,
                T2: std::str::FromStr,
                T1::Err: std::fmt::Debug,
                T2::Err: std::fmt::Debug,
            {
                (self.next(), self.next())
            }
            fn next_string(&mut self) -> Option<String> {
                self.buf.pop().or_else(|| match self.update() {
                    true => self.next_string(),
                    false => None,
                })
            }
            #[inline]
            fn update(&mut self) -> bool {
                let mut s = String::new();
                let res = self.br.read_line(&mut s);
                match res.expect("I/O error.") {
                    0 => false,
                    _ => {
                        self.buf = s.split_whitespace().map(|x| x.to_string()).rev().collect();
                        true
                    }
                }
            }
        }

        #[test]
        fn test_next() {
            let mut sc = new("hoge".as_bytes());
            let s: String = sc.next();
            assert_eq!("hoge", s);
        }
    }
}