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;
using static System.Console;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;
class Program
{
    public static Scanner cin;
    static void Main()
    {
        
        cin = new Scanner();
        long[] nm = cin.arrayLong(2);
        long n = nm[0];
        long m = nm[1];
        long[][] ab = new long[n][];
        for(int a = 0; a < n; a++)
        {
            ab[a] = cin.arrayLong(2);
        }
        ab = ab.OrderBy(i => i[0]).ToArray();
        foreach (long[] a in ab)
        {
            Write("{");
            Write(string.Join(",",a));
            Write("},");
        }
        long total = 0;
        long count = 0;
        for(int a = 0; a < ab.Length; a++)
        {
            for(int b=0;b< ab[a][1]; b++)
            {
                total += ab[a][0];
                count++;
                if (count == m) break;
            }
            if (count == m) break;
        }
        WriteLine(total);
        ReadLine();

    }
}





class Scanner
{
    string[] s;
    int i;

    char[] cs = new char[] { ' ' };

    public Scanner()
    {
        s = new string[0];
        i = 0;
    }

    public string input()
    {
        if (i < s.Length) return s[i++];
        string st = Console.ReadLine();
        while (st == "") st = Console.ReadLine();
        s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        if (s.Length == 0) return input();
        i = 0;
        return s[i++];
    }

    public int inputInt()
    {
        return int.Parse(input());
    }
    public int[] arrayInt(int N, int add = 0)
    {
        int[] Array = new int[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = inputInt() + add;
        }
        return Array;
    }

    public long inputLong()
    {
        return long.Parse(input());
    }

    public long[] arrayLong(int N, long add = 0)
    {
        long[] Array = new long[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = inputLong() + add;
        }
        return Array;
    }

    public double inputDouble()
    {
        return double.Parse(input());
    }


    public double[] arrayDouble(int N, double add = 0)
    {
        double[] Array = new double[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = inputDouble() + add;
        }
        return Array;
    }
}