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::{self, Read};
use std::iter::FromIterator;

trait Source {
    fn next_chr(&mut self) -> char;
    fn next_str(&mut self) -> String;
}

#[allow(dead_code)]
fn from_stdin() -> SourceImpl {
    let mut data = String::new();
    io::stdin().read_to_string(&mut data).expect("Input error");
    from_str(&data)
}

fn from_str(s: &str) -> SourceImpl {
    let data = String::from(s);
    let chars = data.chars().collect::<Vec<char>>().into_iter();
    SourceImpl { chars: chars }
}

struct SourceImpl {
    chars: std::vec::IntoIter<char>,
}

impl Source for SourceImpl {
    fn next_chr(&mut self) -> char {
        self.chars
            .by_ref()
            .skip_while(|c| char::is_whitespace(*c))
            .next()
            .expect("eof")
    }

    fn next_str(&mut self) -> String {
        let it = self
            .chars
            .by_ref()
            .skip_while(|c| char::is_whitespace(*c))
            .take_while(|c| !char::is_whitespace(*c));
        let s = String::from_iter(it);
        if s.is_empty() {
            panic!("eof")
        } else {
            s
        }
    }
}

#[allow(unused_macros)]
macro_rules! input {
    ($src:expr; $($elem:tt)*) => {
        input_inner!($src, $($elem)*)
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($src:expr) => {};
    ($src:expr,) => {};
    ($src:expr, $var:ident : mut $ty:tt $($rest:tt)*) => {
        let mut $var = parse!($src, $ty);
        input_inner!($src $($rest)*)
    };
    ($src:expr, $var:ident : $ty:tt $($rest:tt)*) => {
        let $var = parse!($src, $ty);
        input_inner!($src $($rest)*)
    };
}

#[allow(unused_macros)]
macro_rules! parse {
    ($src:expr, ( $($ty:tt),* )) => {
        ( $(parse!($src,$ty)),* )
    };
    ($src:expr, char) => {
        $src.next_chr()
    };
    ($src:expr, [ $ty:tt; $len:expr ]) => {
        (0..$len).map(|_| parse!($src,$ty)).collect::<Vec<_>>()
    };
    ($src:expr, $ty:tt) => {
        parse_str!($src,$ty)
    };
}

#[allow(unused_macros)]
macro_rules! parse_str {
    ($src:expr, chars) => {
        $src.next_str().chars().collect::<Vec<char>>()
    };
    ($src:expr, $ty:ty) => {
        $src.next_str().parse::<$ty>().expect("Parse error")
    };
}

use std::cmp::min;

fn main() {
    let mut src = from_stdin();
    input! { src;
             n: u64, m: u64,
             ab: mut [(u64,u64); n]
    }

    ab.sort();

    let (_, v) = ab.iter().fold((0, 0), |(c, v), &(a, b)| {
        let k = min(b, m - c);
        (c + k, v + a * k)
    });

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