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: Wrong Answer)


import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {

  public static void main(String[] args) {

      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      int M = sc.nextInt();
      List<List<Integer>> list = new ArrayList<>();
      for (int i = 0; i < N; i++) {
          int key = sc.nextInt();
          int value = sc.nextInt();
          List<Integer> temp = Arrays.asList(key, value);
          list.add(temp);
      }
      list = list.stream().sorted((o1, o2) -> o1.get(0).compareTo(o2.get(0))).collect(Collectors.toList());
      int current = 0;
      int num = M;
      long result = 0;
      while(true) {
          if (list.get(current).get(1) < num) {
              result += list.get(current).get(0) * list.get(current).get(1);
              num -= list.get(current).get(1);
              current++;
          } else {
              result += (long) list.get(current).get(0) * (long) num;
              break;
          }
      }
      System.out.println(result);
  }
}