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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class ABC121C{
    public static void Main(){
        var NM = Console.ReadLine().Split().Select(int.Parse).ToList();
        var A = new List<int>();
        var B = new List<int>();
        foreach(var n in Enumerable.Range(0,NM[0])){
            var AB = Console.ReadLine().Split().Select(int.Parse).ToList();
            A.Add(AB[0]);
            B.Add(AB[1]);
        }
        var yen = 0;
        var count = 0;
        var min = B.FindIndex(n=>n==B.Min());
        while(true){
            count += B[min];
            yen += A[min]*B[min];
            if(count>=NM[1]){
                yen -= (count-NM[1])*A[min];
                break;
            }
            B[min] = int.MaxValue;
            min = B.FindIndex(n=>n==B.Min());
        }
        System.Console.WriteLine(yen);
    }
}