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.Scanner;
import java.util.TreeMap;

public class Main {


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long M = sc.nextLong();
		long Ai = 0;
		long Bi = 0;
		
		
		TreeMap <Long, Long> store = new TreeMap <Long, Long>();
		
		for (int i = 0; i<N; i++){
			Ai = sc.nextLong();
			Bi = sc.nextLong();
 
			if (store.containsKey(Ai)) 
				store.put(Ai, store.get(Ai) + Bi);
			 else 
				store.put(Ai, Bi);
			}
		
		sc.close();
		
		long price = 0;
		long buy = 0;
		
		for (Long cost : store.keySet()) {
			buy = Math.min(M, store.get(cost));
			price = price + cost*buy;
			M=M-buy;
			
		}
		System.out.println(price);
			
	}
}