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{
  public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    String[] S = sc.nextLine().split(" ");
    int N = Integer.parseInt(S[0]);
    int M = Integer.parseInt(S[1]);
    Map<Integer, Integer> map = new TreeMap<>();
    int[][] val = new int[N][2];
    for(int i = 0; i < N; i++){
      String[] T = sc.nextLine().split(" ");
      int a = Integer.parseInt(T[0]);
      int b = Integer.parseInt(T[1]);
      map.put(a, b);
      val[i][0] = a;
      val[i][1] = b;
    }
    Arrays.sort(val, (a, b) -> Integer.compare(a[0], b[0]));
    long ans = 0;
    for(int i = 0; M > 0; i++){
      long a = val[i][0];
      long b = val[i][1];
      if(val[i][1] < M){        
        ans += a * b;
        M -= b;
      }else{
        ans += a * M;
        M = 0;
      }
    }
    System.out.println(ans);
  }
}