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 sc = new Scanner(System.in);
        int N = sc.nextInt();
        long M = sc.nextLong();
        long A[] = new long[N];
        long B[] = new long[N];
        List<Store> ls = new ArrayList<>();
        for(int i=0; i<N; i++) {
            A[i] = sc.nextLong();
            B[i] = sc.nextLong();
            Store s = new Store(A[i], B[i]);
            ls.add(s);
        }
        Collections.sort(ls);
        long r=0;
        for(Store s : ls) {
//            System.out.println(String.format("%d:%d", s.price, s.stock));
            if(M>s.stock) {
                r+=s.stock*s.price;
                M-=s.stock;
                continue;
            }
            r+=M*s.price;
            break;
        }
        System.out.println(r);
   }
   static class Store implements Comparable {
        long price;
        long stock;
        Store(long p, long s) {
            this.price = p;
            this.stock = s;
        }

       @Override
       public int compareTo(Object o) {
           Store t = (Store)o;
           return (this.price>t.price ? 1 : -1);
       }
   }
}