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;

public class Program
{

	public static void Main(string[] args)
	{
		// スペース区切りの整数の入力
		var input = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();

		var N = input[0];
		var M = input[1];

		var shopDic = new Dictionary<long, long>();
		for (int i = 0; i < N; i++)
		{
			input = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
			// 既にキーが追加されていたら値だけ追加
			if (shopDic.ContainsKey(input[0]))
			{
				shopDic[input[0]] += input[1];
			}
			else
			{
				shopDic.Add(input[0], input[1]);
			}
		}
		shopDic = shopDic.OrderBy(x => x.Key).ToDictionary(x => x.Key, x => x.Value);

		long answer = 0;
		foreach (KeyValuePair<long, long> pair in shopDic)
		{
			// 次の店に行く必要がない場合
			if (M <= pair.Value)
			{
				answer += pair.Key * M;
				break;
			}
			else
			{
				answer += pair.Key * pair.Value;
				M -= pair.Value;
			}
		}

		Console.WriteLine(answer);
	}
}