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 (1.42.0) (judge verdict: Accepted)

use proconio::{input, fastout};

fn cmp(ff: &(usize, usize), ll: &(usize, usize)) -> std::cmp::Ordering {
    use std::cmp::Ordering::*;
    if ff.0 < ll.0 { Less }
    else if ff.0 == ll.0 { Equal }
    else { Greater }
}

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

    let mut arr = ab.clone();
    arr.sort_by(cmp);
    let mut res = m;
    let mut ans = 0;
    for (a, b) in arr {
        if res <= b {
            ans += a*res;
            break;
        } else {
            ans += a*b;
            res -= b;
        }
    }
    println!("{}", ans);
}