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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main {

    static Scanner sc = new Scanner(System.in);

    public static void main(String[] args) {
        int N = sc.nextInt();
        int M = sc.nextInt();

        TreeMap<Long, Integer> priceToStock = new TreeMap<>();

        for (int i = 0; i < N; i++) {
            long price = sc.nextLong();
            int stock = sc.nextInt();
            if (priceToStock.containsKey(price)) {
                int prevStock = priceToStock.get(price);
                priceToStock.put(price, stock + prevStock);
            } else {
                priceToStock.put(price, stock);
            }
        }
        long ans = 0;
        for (Map.Entry<Long, Integer> entry: priceToStock.entrySet()) {
            long price = entry.getKey();
            int stock = entry.getValue();
            if (M > stock) {
                //終わらない
                M -= stock;
                ans += price * (long) stock;
            } else {
                //終わり
                ans += price * M;
                break;
            }
        }

        System.out.println(ans);
    }
}