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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;

class Main{
    static Scanner sc = new Scanner(System.in);
    static StringBuffer sb = new StringBuffer();
    public static void main(String args[]){
        int N=sc.nextInt(),M=sc.nextInt(),i=0,c;
        long a;
        TreeMap<Integer,Long> cost=new TreeMap<>();
        for(i=0;i<N;i++){
            c=sc.nextInt();
            a=sc.nextInt();
            if(!cost.containsKey(c)){
                cost.put(c,a);
            }else{
                cost.put(c,cost.get(c)+a);
            }
        }
        a=0;
        for(int k:cost.keySet()){
            long m=Math.min(M,cost.get(k));
            a+=k*m;
            M-=m;
            if(M==0)break;
        }
        System.out.println(a);
    }
}