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.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        int a = Integer.parseInt(scanner.next());
        int b = Integer.parseInt(scanner.next());
        int[][] r = new int[a][2];
        for(int x = 0; x < a; x++) {
            r[x][0] = Integer.parseInt(scanner.next());
            r[x][1] = Integer.parseInt(scanner.next());
        }
        Comp comp = new Comp();
        comp.set_index(1);
        Arrays.sort (r, comp);
        long count = 0;
        for(int x = 0;; x++){
            if(b < 0) {
                System.out.println("a" + x);
                break;
            }
            b = b - r[x][1];
            count +=  b > 0 ? (r[x][0] * r[x][1]) : r[x][0] * (r[x][1] - Math.abs(b));
        }

        System.out.println(count);
    }
}
class Comp implements Comparator {

    int index = 0;

    void set_index (int i) {
        index = i;
    }

    public int compare(Object a, Object b) {
        int[] int_a = (int[]) a;
        int[] int_b = (int[]) b;
        return (int_a[index] - int_b[index]);
    }

}