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.Collections.Generic;

namespace MyProgram{
    class AtCoder{
        static void Main(string[] args){
            long[] input = Array.ConvertAll(Console.ReadLine().Split(' '),long.Parse);
            long N = input[0];
            long M = input[1];
            List<Shop> shop = new List<Shop>();
            
            for(var i=0;i<N;i++){
                long[] input1 = Array.ConvertAll(Console.ReadLine().Split(' '),long.Parse);
                Shop s = new Shop();
                s.A = input1[0];
                s.B = input1[1];
                shop.Add(s);
            }
            shop.Sort(delegate(Shop a,Shop b){return (int)(a.A-b.A);});
        
            long sum_money=0;
            long count=0;
            for(var i=0;i<N;i++){
                for(var j=0;j<shop[i].B;j++){
                    sum_money += shop[i].A;
                    count++;
                    if(count>=M)goto END;
                }
            }
            END:
            Console.WriteLine(sum_money);
        }
    } 
    public class Shop{
        public long A;
        public long B;
    }
}