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.*;

public class Main{
    public static void main(String[]args){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int m=sc.nextInt();
        long a[]=new long[n];
        int b[]=new int[n];
        Map<String,Integer>map=new HashMap<>();
        for(int i=0;i<n;i++){
            a[i]=sc.nextLong();
            b[i]=sc.nextInt();
            map.put(String.valueOf(a[i]),b[i]);
        }
        Arrays.sort(a);
        long total=0;
        for(int i=0;i<n;i++){
            if(map.containsKey(String.valueOf(a[i]))){
                if(m>map.get(String.valueOf(a[i]))){
                    m-=map.get(String.valueOf(a[i]));
                    total+=a[i]*map.get(String.valueOf(a[i]));
                }else{
                    total+=m*a[i];
                    break;
                }
            }
        }
        System.out.println(total);

    }
}