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::stdin;
use std::time::SystemTime;

fn main() {
    let nm: Vec<usize> = read().split(" ").map(|n| n.parse().unwrap()).collect();
    let mut vec: Vec<(usize, usize)> = vec![];
    for _ in 0..nm[0] {
        let ab: Vec<usize> = read().split(" ").map(|n| n.parse().unwrap()).collect();
        vec.push((ab[0], ab[1]));
    }
    vec.sort_by(|a, b| a.0.cmp(&b.0));
    
    let mut yen = 0;
    let mut count = 0;
    for v in vec {
        if count == nm[1] {
            break;
        }
        
        let t = nm[1] - count;
        if v.1 <= t {
            count = count + v.1;
            yen = yen + v.0 * v.1;
        } else {
            count = count + t;
            yen = yen + v.0 * t;
        }
    }
    println!("{}", yen);
}

fn read() -> String {
    let mut s = String::new();
    stdin().read_line(&mut s).unwrap();
    s.trim().to_string()
}