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: Runtime Error)

import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner scn = new Scanner(System.in);
    int N = scn.nextInt();
    int M = scn.nextInt();

    ArrayList<Data> data = new ArrayList<>();
    for (int i = 0; i < N; i++) {
      int A = scn.nextInt();
      int B = scn.nextInt();
      data.add(new Data(A, B));
    }

    Collections.sort(data, new DataComparator());

    int expense = 0;
    int drinkCount = 0;
    for (Data d : data) {
      drinkCount += d.stock;
      expense += d.stock * d.price;

      if (drinkCount >= M) {
        int excess = drinkCount - M;
        expense -= excess * d.price;
        break;
      }
    }

    System.out.println(expense);
  }

  public static class Data {
    private int price;
    private int stock;

    Data(int a, int b) {
      this.price = a;
      this.stock = b;
    }

    @Override
    public String toString() {
      return String.valueOf(this.price) + ":" + String.valueOf(this.stock);
    }
  }

  public static class DataComparator implements Comparator<Data> {
    @Override
    public int compare(Data d1, Data d2) {
      return d1.price < d2.price ? -1 : 1;
    }
  }

}