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.*;

class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    int m=sc.nextInt();
    I[] a=new I[n];
    for(int i=0;i<n;i++){
      a[i]=new I(sc.nextInt(),sc.nextInt());
    }
    Arrays.sort(a,(s1,s2)->s1.p-s2.p);
//    System.out.println(Arrays.toString(a));
    long k=0;
    long c=0;
    for(int i=0;i<n;i++){
//      System.out.println(a[i]);
      if(a[i].k+k>=m){
        c+=(m-k)*a[i].p;
        break;
      }else{
        k+=a[i].k;
        c+=(long)(a[i].k)*(long)(a[i].p);
      }
//      System.out.println("k"+k+" c"+c);
    }
    System.out.println(c);
  }
  static class I{
    int p;
    int k;
    I(int p,int k){
      this.p=p;
      this.k=k;
    }
    public String toString(){
      return String.valueOf(p)+":"+String.valueOf(k);
    }
  }
}