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.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        long n = sc.nextLong();
        long m = sc.nextLong();
        ArrayList<Store> sl = new ArrayList<>();
        for(int i = 0; i < n; i++){
            long a = sc.nextLong();
            long b = sc.nextLong();
            Store s = new Store(a,b);
            sl.add(s);
        }
        Collections.sort(sl);
        long ans = 0;
        for(Store s : sl){
            if(m <= s.b){
                ans += m * s.a;
                break;
            }else{
                ans += s.b * s.a;
                m -= s.b;
            }
        }
        System.out.println(ans);
    }
}

class Store implements Comparable<Store>{
    long a;
    long b;
    public Store(long a, long b){
        this.a = a;
        this.b = b;
    }
    public int compareTo(Store s2){
        if(this.a > s2.a){
            return 1;
        }else if(this.a < s2.a){
            return -1;
        }else{
            return 0;
        }
    }
}