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;

namespace AtCoderCore {
	public class Program {
		private const int MOD = 1000000007;

		static void Main (string[] args) {
			int[] nm = Console.ReadLine ().Split (' ').Select (x => int.Parse (x)).ToArray ();
			int n = nm[0];
			int m = nm[1];
			AB[] ab = new AB[n];
			for(int i = 0; i < n; ++i){
				int[] temp = Console.ReadLine ().Split (' ').Select (x => int.Parse (x)).ToArray ();
				ab[i] = new AB{A = temp[0], B = temp[1]};
			}

			var ab2 = ab.OrderBy(x => x.A).ToArray();
			long ans = 0;
			for(int i = 0; i < n; ++i){
				if(m >= ab2[i].B){
					m -= ab2[i].B;
					ans += (long)ab2[i].A * ab2[i].B;
				}else {
					ans += (long)ab2[i].A * m;
				}

				if(m <= 0){
					break;
				}
			}

			Console.WriteLine(ans);
		}

		public class AB
		{
			public int A {get; set;}
			public int B {get; set;}
		}
	}
}