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.Linq;
using System.Collections.Generic;


namespace _121c
{
    class Program
    {
        static void Main(string[] args)
        {
            ulong[] read = Array.ConvertAll(Console.ReadLine().Split(),ulong.Parse);
            Dictionary<ulong,ulong> AB = new Dictionary<ulong, ulong>();
            ulong[] readLong = new ulong[2];
            for(ulong i = 0; i < read[0]; i++){
                readLong = Array.ConvertAll(Console.ReadLine().Split(),ulong.Parse);
                AB.Add(readLong[0],readLong[1]);
            }
            var sortrdAB = AB.OrderBy(x=>x.Key);
            ulong count = 0;
            ulong sum = 0;
            foreach(var tekito in sortrdAB){
                if(count+tekito.Value<=read[1]){
                    sum += tekito.Key * tekito.Value;
                    count += tekito.Value;
                }else{
                    sum += tekito.Key * (read[1]-count);
                    break;
                }
            }
            Console.WriteLine(sum);
        }
    }
}