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 std::io::prelude::*;
macro_rules! read_line {
    () => {{
        let mut input = String::new();
        std::io::stdin().read_line(&mut input).unwrap();
        input
    }};
}
pub fn read_to_vec<T: std::str::FromStr>() -> Vec<T> {
    read_line!().split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}
macro_rules! bind_from_vec {
    ( $($variable:ident),+ = $vector:expr ) => {
        let mut v = $vector.clone().into_iter();
        $(let $variable = v.next().unwrap();)+
    };
}
fn read_two<T: std::str::FromStr>(stdin: &mut std::io::StdinLock, buf: &mut String) -> (T, T) {
    buf.clear();
    stdin.read_line(buf).ok();
    let mut iter = buf.split_whitespace().map(|e| e.parse().ok().unwrap());

    (iter.next().unwrap(), iter.next().unwrap())
}
fn main() {
    bind_from_vec!(n, m = read_to_vec());
    let mut m = m;

    let mut input = Vec::with_capacity(n as usize);
    {
        let stdin = std::io::stdin();
        let mut locked_stdin = stdin.lock();
        let mut buf = String::new();

        for _ in 0..n {
            let (a, b) = read_two::<u64>(&mut locked_stdin, &mut buf);
            input.push((a, b));
        }
    }
    input.sort_by_key(|e| e.0);

    let mut ans = 0;
    for &(a, b) in input.iter() {
        if m > b {
            ans += a * b;
            m -= b;
        } else {
            ans += a * m;
            break;
        }
    }

    println!("{}", ans);
}