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.*;
import java.util.Arrays;
import java.util.Comparator;
public class Main {    
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();        
		int m = sc.nextInt();       
        Data [] d = new Data [n];
        for(int i=0;i<n;i++){int A = sc.nextInt();
                             int B = sc.nextInt();
                             d[i] = new Data(A,B);                            
                            }
        Arrays.sort(d, new Comparator<Data>() {
            public int compare(Data d1, Data d2) {
                return d1.getA() - d2.getA();
            }
        }); 
        long ans = 0;
        int count1 = 0;
        int count2 = 0;
        int point = 0;
        while(count1<m){ans+=d[point].getA();
                        count1++;
                        count2++;
                        if(count2==d[point].getB()){count2=0;
                                                    point++;}
                      }
        System.out.println(ans);
	}
}
class Data{
    private int a;
    private int b;
    public Data(int a, int b) {
        this.a = a;
        this.b = b;
    }
    public int getA() {
        return a;
    }
    public int getB() {
        return b;
    }
}