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.Text;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Threading.Tasks;
class Program
{
	static void Main(string[] args)
	{
		var input = Console.ReadLine().Split().Select(x => long.Parse(x)).ToArray();
		long N = input[0];
		long M = input[1];
		List<Tuple<long, long>> AB = new List<Tuple<long, long>>();
        for(var i = 0; i < N; i++)
		{
			var abin = Console.ReadLine().Split().Select(x => long.Parse(x)).ToArray();
			AB.Add(new Tuple<long, long>(abin[0], abin[1]));
		}
		AB.Sort((c, d) => ((int)c.Item1 - (int)d.Item1));
		long ans = 0;
		long cnt = 0;
        for(var i = 0; i < N; i++)
		{
			if (cnt + AB[i].Item2 <= M)
			{
				cnt += AB[i].Item2;
				ans += AB[i].Item1 * AB[i].Item2;
			}
			else
			{
				ans += (M - cnt) * AB[i].Item1;
				break;
			}
		}

        Console.WriteLine(ans);

	}
}