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.Generic;
using System.Linq;

public class MainClass
{
	public static void Main(string[] args)
	{
		string[] inputs = Console.ReadLine().Split(' ');
		int N = int.Parse(inputs[0]);
		long m = long.Parse(inputs[1]);
		
		List<Tuple<int,int>> a = new List<Tuple<int,int>>(N);
		
		for(int i = 0; i < N; i++)
		{
			inputs = Console.ReadLine().Split(' ');
			a.Add(new Tuple<int,int>(int.Parse(inputs[0]), int.Parse(inputs[1])));
		}
		
		a.Sort((e1,e2) => e1.Item1 - e2.Item1);
		
		long yen = 0;
		long katta = 0;
		
		for(int i = 0 ;katta < m;)
		{
			int max = a[i].Item2;
			if(katta + max >= m)
			{
				long k = m - katta;
				katta += k;
				yen += k * a[i].Item1;
				break;
			}
			else
			{
				katta += max;
				yen += max * a[i].Item1;
				i++;
			}
		}
		Console.WriteLine(yen);
	}
}