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

namespace AtCoder
{
    class Store
    {
        public long price;
        public long num;
        public long sum;
    }

    class Program
    {

        static void Main()
        {
            //ここから
            int[] str = ReadLineToIntArray();
            int N = str[0];
            int M = str[1];

            List<Store> stores = new List<Store>();

            for(int i = 0; i < N; i++)
            {
                stores.Add(new Store());
                long[] temp = ReadLineToLongArray();
                stores[i].price = temp[0];
                stores[i].num = temp[1];
                stores[i].sum = temp[0] * temp[1];
            }

            stores = stores.OrderBy(s => s.price).ToList();

            long sum = 0, ans = 0;

            while (sum < M)
            {
                foreach(var store in stores)
                {
                    if (sum + store.num <= M)
                    {
                        sum += store.num;
                        ans += store.sum;
                    }
                    else
                    {
                        ans += store.price * (M - sum);
                        sum += M - sum;
                        break;
                    }
                }
            }

            //ここまで


            Console.WriteLine(ans);
            Console.ReadKey();
        }


        static public void Swap<T>(ref T a,ref T b) { T temp = a;a = b;b = temp; }
        static public long GCD(int a, int b){ return b != 0 ? GCD(b, a % b) : a;}
        static public long LCM(int a, int b){ return (long)a * b / GCD(a, b); }



        
        static public string ReadLineToString(){return Console.ReadLine();}
        static public int ReadLineToInt(){return int.Parse(Console.ReadLine());}
        static public long ReadLineToLong() { return long.Parse(Console.ReadLine()); }
        static public string[] ReadLineToStringArray(){return Console.ReadLine().Split(' ').ToArray();}
        static public int[] ReadLineToIntArray(){return Console.ReadLine().Split(' ').Select(int.Parse).ToArray();}
        static public long[] ReadLineToLongArray() { return Console.ReadLine().Split(' ').Select(long.Parse).ToArray(); }
        static public List<string> ReadLineToStringList(){return Console.ReadLine().Split(' ').ToList();}
        static public List<int> ReadLineToIntList(){return Console.ReadLine().Split(' ').Select(int.Parse).ToList();}
        static public List<long> ReadLineToLongList() { return Console.ReadLine().Split(' ').Select(long.Parse).ToList(); }



    }
}