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(non_snake_case)]

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

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read_line()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

fn read_int_tuple() -> (i32, i32) {
    let v: Vec<i32> = read_vec();
    (v[0], v[1])
}

struct Store {
    price: i64,
    amount: i64
}

fn main() {
    let (N, M) = read_int_tuple();
    let N = N as usize;
    let mut M = M as i64;
    let mut stores = Vec::new();
    for _ in 0..N {
        let (A, B) = read_int_tuple();
        stores.push(Store {
            price: A as i64,
            amount: B as i64
        });
    }
    stores.sort_by(|a, b| a.price.cmp(&b.price));
    let mut ans: i64 = 0;
    for store in stores {
        if store.amount > M {
            ans += (store.price * M);
            break;
        } else {
            ans += store.price * store.amount;
            M -= store.amount;
        }
    }
    println!("{}", ans);
}