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: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;


public class Store
{
	public long A { get; private set; }
	public long B { get; private set; }
	
	public Store(long a, long b)
	{
		A = a;
		B = b;
	}
}


public class C
{
	public static void Main()
	{
		string[] args;

		args = Console.ReadLine().Split(' ');
		int N = Int32.Parse(args[0]);
		int M = Int32.Parse(args[1]);
		
		List<Store> storeList = new List<Store>(N);
		for(int i = 0; i < N; i++)
		{
			args = Console.ReadLine().Split(' ');
			long a = Int32.Parse(args[0]);
			long b = Int32.Parse(args[1]);
			storeList.Add(new Store(a,b));
		}
		storeList.Sort((x,y) => (int)(x.A - y.A));
		
		long rest = M;
		long ans = 0;
		int index = 0;
		while(rest > 0)
		{
			Store current = storeList[index];
			index++;
			long a = current.A;
			long b = current.B;
			
			long numBuy = Math.Min(rest, b);
			ans += numBuy * a;
			rest -= numBuy;			
		}		
		
		Console.WriteLine(ans);
	}	
}