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)

#![allow(unused_mut, unused_macros, unused_variables, unused_assignments, non_snake_case, unused_imports, dead_code)]

use std::collections::{HashMap, BinaryHeap, BTreeMap};

// [Rustで競技プログラミングの入力をスッキリ記述するマクロ - Qiita]
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8 から拝借
macro_rules! input {
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}
macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}
macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };
    // ABC128-C-Switches用に拡張
    ($next:expr, [ $t:tt ]) => {{
        let len = read_value!($next, $t);
        (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    }};
    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };
    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };
    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

// 十分に理解した、可愛いお手製のライブラリ達
macro_rules! p {
    ($t:expr) => {
        println!("{}", $t);
    };
}
macro_rules! debug {
    ($t:expr) => {
        println!("{:?}", $t);
    };
}
fn gcd(a:usize, b:usize) -> usize {
    if b == 0 {return a};
    gcd(b, a % b)
}
fn lcm(a:usize, b:usize) -> usize {
    a / gcd(a, b) * b
}

// エントリーポイント
fn main() {
    input! {
        N:usize,
        M:usize,
        AB:[(usize, usize);N]
    }
    let items = {
        let mut items = BTreeMap::new();
        for ab in AB {
            let (a, b) = ab;
            if !items.contains_key(&a) {
                items.insert(a, b);
            } else {
                *items.get_mut(&a).unwrap() += b;
            }
        }
        items
    };
    let (mut m, mut cost) = (M, 0usize);
    for (price, count) in items {
        if m < count {
            cost += price * m;
            break;
        }
        cost += price * count;
        m -= count;
    }
    p!(cost);
}