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.Arrays;
import java.util.Comparator;
import java.util.Scanner;

class Main {

  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    int n = scanner.nextInt();
    int m = scanner.nextInt();
    Data[] datas = new Data[n];
    for (int i = 0; i < n; i++) {
      long a = scanner.nextLong();
      long b = scanner.nextLong();
      datas[i] = new Data(a, b);
    }
    Arrays.sort(datas, new Comparator<Data>() {
      @Override
      public int compare(Data d1, Data d2) {
        return (int) (d1.getPrice() - d2.getPrice());
      }
    });

    long sum = 0;
    long money = 0;

    for (int i = 0; i < n; i++) {
      if (sum + datas[i].getStock() >= m) {
        money += datas[i].getPrice() * (m - sum);
        break;
      } else {
        sum += datas[i].getStock();
        money += datas[i].getPrice() * datas[i].getStock();
      }
    }
    System.out.println(money);
  }

}

class Data {
  private long a;
  private long b;

  public Data(long a, long b) {
    this.a = a;
    this.b = b;
  }

  public long getPrice() {
    return a;
  }

  public long getStock() {
    return b;
  }
}