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.math.BigInteger;
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();
		ArrayList<Abc> abl = new  ArrayList<>(n);
		int i;
		for(i = 0; i < n; i++) {
			Abc ab = new Abc(sc.nextLong(), sc.nextLong());
			abl.add(ab);
		}
		sc = null;
		Collections.sort(abl, new Abcmp());
		BigInteger yen = new BigInteger("0");
		//long yen = 0;
		long m1 = m;
		for(i = 0; i < n; i++) {
			if(m1 >= abl.get(i).b) {
				yen = yen.add(new BigInteger(Long.toString(abl.get(i).a * abl.get(i).b)));
				//yen += abl.get(i).a * abl.get(i).b;
				m1 -= abl.get(i).b;
			} else {
				yen = yen.add(new BigInteger(Long.toString(abl.get(i).a * m1)));
				//yen += abl.get(i).a * m1;
				break;
			}
		}
		System.out.println(yen);
	}
}

class Abc {
	long a;
	long b;
	Abc(long a, long b) {
		this.a = a;
		this.b = b;
	}
}

class Abcmp implements  Comparator<Abc> {
	@Override
	public int compare(Abc p1, Abc p2) {
		return p1.a < p2.a ? -1 : 1;
	}
}