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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Atcoder
{
    class AtCodeeeeeeeeeeeeeeeeeeeer
    {
        static void Main(string[] args)
        {
            int[] s = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            int N = s[0];
            int M = s[1];
            int[,] input = new int[N, 2];
            int now = 0;long ans = 0;
            for (int i = 0; i < N; i++)
            {
                int[] inputs = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                for (int j = 0; j < 2 ; j++)
                {
                    input[i, j] = inputs[j];
                } 
            }

            while (true)
            {
                if (now < M)
                {
                    //Aiの最小を求める
                    int AiMin = int.MaxValue;
                    int I = 0;
                    for (int i = 0; i < N; i++)
                    {
                        if (input[i, 0] < AiMin)
                        {
                            AiMin = input[i, 0];
                            I = i;
                        }
                    }
                    //一番安いのを一本買う
                    ans += input[I, 0];
                    now++;
                    input[I, 1] -= 1;
                    if (input[I, 1] == 0)
                    {
                        input[I, 0] = 1000000;//売り切れならバカ高くする
                    }
                }
                else
                {
                    Console.WriteLine(ans);
                    break;
                }
            }
            


        }
    }
}