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: Wrong Answer)

import java.util.HashMap;
	import java.util.Map;
	import java.util.Map.Entry;
	import java.util.Scanner;
	import java.util.TreeMap;

	public class Main{

		public static void main(String[] args) {

			Scanner scanner = new Scanner(System.in);

			Map<Integer,Integer> list = new HashMap<Integer,Integer>();

			String[] temp = scanner.nextLine().split(" ");
			int n = Integer.parseInt(temp[0]);
			int m = Integer.parseInt(temp[1]);
			int t1 = 0;
			int t2 = 0;
			for(int i = 0; i < n; i ++) {
				temp = scanner.nextLine().split(" ");
				t1 = Integer.parseInt(temp[0]);
				t2 = Integer.parseInt(temp[1]);
				if(list.containsKey(t1)) {

					t2 = list.get(t1);
					list.put(t1, t2);
				}else {
					list.put(t1, t2);
				}
			}
			TreeMap<Integer, Integer> sorted = new TreeMap<>(list);

			long ans = 0;
			int kakaku = 0;
			int kazu = 0;
			int flag = m;
			for(Entry<Integer,Integer> entry : sorted.entrySet()) {
				kakaku = entry.getKey();
				kazu = entry.getValue();

				for(int i = 0; i < kazu ; i++) {
					flag--;
					ans += kakaku;
					if(flag == 0) {
						break;
					}
				}
				if(flag == 0) {
					break;
				}
			}

			System.out.println(ans);


		}

	}