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)

macro_rules! read_line {
    () => {{
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim_right().to_owned()
    }};
}

macro_rules! parse_next {
    ($ws:expr) => {
        $ws.next().unwrap().parse().unwrap();
    };
}

fn main() {
    let (n, m) = {
        let line = read_line!();
        let mut ws = line.split_whitespace();
        let n: usize = parse_next!(ws);
        let m: i32 = parse_next!(ws);
        (n, m)
    };
    let mut ab = {
        let mut ab = vec![(0i64, 0); n];
        for ni in 0..n {
            let line = read_line!();
            let mut ws = line.split_whitespace();
            ab[ni] = (parse_next!(ws), parse_next!(ws));
        }
        ab
    };

    ab.sort();

    let mut left = m;
    let mut ans: i64 = 0;
    for (a, b) in ab {
        let x = std::cmp::min(b, left);
        ans += a * (x as i64);
        left -= x;
        if left == 0 {
            break;
        }
    }

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