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.Linq;
using System.Collections;
using System.Collections.Generic;

    class V
    {
        public int p;
        public int m;
    }

class Program
{
    public static void Main()
    {
      int N, M;
        string[] strs = Console.ReadLine().Split(' ');
        N = int.Parse(strs[0]);
        M = int.Parse(strs[1]);
        List<V> L = new List<V>() { Capacity = N };
        for(int i = 0; i < N; i++)
        {
            strs = Console.ReadLine().Split(' ');
            V tmp = new V();
            tmp.p = int.Parse(strs[0]);
            tmp.m = int.Parse(strs[1]);
            L.Add(tmp);
        }
        L.Sort((x, y) => x.p - y.p);
        
        int price = 0;
        for(int i = 0; i < N; i++)
        {
            int c = Math.Min(L[i].m, M);
            price += c * L[i].p;
            L[i].m -= c;
            M -= c;
            if (M <= 0)
                break;
        }
        Console.WriteLine(price);
    }
}