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: Runtime Error)

using System;
using System.Collections.Generic;

class Program
{
	static void Main(string[] args)
	{
		string[] str = Console.ReadLine().Split(' ');
		int N = int.Parse(str[0]);
		int M = int.Parse(str[1]);

		List<int> A = new List<int>();
		List<int> B = new List<int>();
		SortedDictionary<int, int> AB = new SortedDictionary<int, int>();
		for (int i = 0; i < N; i++)
		{
			string[] str2 = Console.ReadLine().Split(' ');
			AB.Add(int.Parse(str2[0]), int.Parse(str2[1]));
		}
		//入力完了

		int price = 0;
		int cnt = 0;
		foreach (var n in AB)
		{
			// Console.WriteLine(n.Key + " : " + n.Value);
			A.Add(n.Key);
			B.Add(n.Value);
		}
		int z;
		for (z = 0; z < N; z++)
		{
			for (int j = 1; j <= B[z]; j++)
			{
				cnt++;
				price += A[z];
				if (cnt == M) break;
				// Console.WriteLine(cnt + " : " + price);
			}
		}
		Console.WriteLine(price);
	}
}