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::Read;
use std::str::SplitWhitespace;
use std::str::FromStr;

pub fn read_stdin() -> String {
    let mut s = String::new();
    ::std::io::stdin().read_to_string(&mut s).unwrap();
    s
}

pub trait Parse<T> {
    fn parse(&mut self) -> T;
}

impl<'a, T> Parse<T> for SplitWhitespace<'a>
where
    T: FromStr,
{
    fn parse(&mut self) -> T {
        let word = self.next().unwrap();
        word.parse().ok().unwrap()
    }
}

fn main() {
    let source = read_stdin();
    let mut iter = source.split_whitespace();
    let n: usize = iter.parse();
    let m: u64 = iter.parse();
    let mut v: Vec<(u64, u64)> = (0..n).map(|_| (iter.parse(), iter.parse())).collect();
    v.sort();

    let mut c = 0;
    let mut ans = 0;
    for (a, b) in v {
        let min = std::cmp::min(m - c, b);
        ans += a * min;
        c += min;
        if c == m {
            break;
        }
    }
    println!("{}", ans);
}