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.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace algorithm
{
	class Program
	{
		public class T
		{

			public Int64 cost;
			public Int64 num;
			public T(Int64 cost, Int64 num)
			{
				this.cost = cost;
				this.num = num;
			}
		}
		static void Main(string[] args)
		{	//件数/本数
			Int64[] q = Array.ConvertAll(Console.ReadLine().Split(), Int64.Parse);

			//コスト/本数


			List<T> qq = new List<T>();

			for (Int64 i = 0; i < q[0]; i++)
			{
				Int64[] q2 = new Int64[2];
				q2 = Array.ConvertAll(Console.ReadLine().Split(), Int64.Parse);
				qq.Add(new T(q2[0], q2[1]));
			}


			

			var qqq = qq.OrderBy(x => x.cost);
			Int64 ans=0;
			foreach (var tmp in qqq)
			{
				if (tmp.num <= q[1])
				{
					q[1] -= tmp.num;
					ans += tmp.num * tmp.cost;
				}
				else
				{
					//最後
					ans += q[1] * tmp.cost;
					break;
				}
			}

			//コストが低い順から取得していく



			Console.WriteLine(ans);
			
		}
	}

}