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.Linq;

class Plogram
{
    static void Main()
    {
        var n = Console.ReadLine().Split();
        long N = long.Parse(n[0]);
        long M = long.Parse(n[1]);

        AB[] ab = new AB[N];

        for (int i = 0; i < N; i++)
        {
            var line = Console.ReadLine().Split();
            ab[i] = new AB(long.Parse(line[0]), long.Parse(line[1]));
        }

        var ab2 = ab.OrderBy(x=>x.A).ToArray();
        long count = 0;

        for (int i = 0; i < N; i++)
        {
            if(M >= ab2[i].B)
            {
                M -= ab2[i].B;
                count += ab2[i].A * ab2[i].B;
            }
            else
            {
                count += ab2[i].A * M;
                M = 0;
            }

            if(M<=0)
            {
                break;
            }
        }
        Console.WriteLine(count);
    }
}

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

    public AB(long a, long b)
    {
        A = a;
        B = b;
    }
}