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);
    	int N = sc.nextInt();
    	int M = sc.nextInt();
    	Map<Long, Long> shop = new HashMap<>();
    	for (int i = 0; i < N; i++) {
    		long cost = sc.nextInt();
    		long count = sc.nextInt();
    		if (shop.containsKey(cost)) {
    			shop.put(cost, shop.get(cost) + count);
    		} else {
    			shop.put(cost, count);
    		}
    	}
    	List<Long> key = new ArrayList<>(shop.keySet());
    	Collections.sort(key);
    	long rem = M;
    	long totalCost = 0;
    	for (int i = 0; i < key.size(); i++) {
    		long cost = key.get(i);
    		long count = shop.get(cost);
    		if (count < rem) {
    			rem -= count;
    			totalCost += count * cost;
    		} else {
    			totalCost += rem * cost;
    			break;
    		}
    	}
    	System.out.println(totalCost);
    }
}