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: Time Limit Exceeded)

import java.util.*;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Collections;
import java.awt.Point;


public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] nm = br.readLine().split(" ");
        int n = Integer.parseInt(nm[0]);
        int m = Integer.parseInt(nm[1]);
        
        AB[] abs = new AB[n];
        for (int i = 0; i < n; i++) {
            String[] sab = br.readLine().split(" ");
            int a = Integer.parseInt(sab[0]);
            int b = Integer.parseInt(sab[1]);
            AB ab = new AB();
            ab.a = a;
            ab.b = b;
            abs[i] = ab;
        }
        
        Arrays.sort(abs, new Comparator<AB>(){
            public int compare(AB ab1, AB ab2){
              return ab1.a - ab2.a;
            }
        });
        
        long cost = 0;
        int tobuy = m;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < abs[i].b; j++) {
                if (tobuy > 0) {
                    cost += abs[i].a;
                    tobuy--;
                }
            }
        }
        
        System.out.println(cost);
    }
}

class AB {
    int a;
    int b;
}