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 atcorder.DataStructure;
namespace atcorder
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            atcorder.Test.Run();
        }
    }


    public class Test
    {
        public static void Run()
        {
            Input input=new Input(' ');
            input.ReadLine();
            int N = input.Next<int>();
            int M = input.Next<int>();
            List<Pair<long, long>> pairs = new List<Pair<long, long>>();
            for (int i = 0; i < N;i++){
                Pair<long, long> pair = new Pair<long, long>();
                input.ReadLine();
                pair.first = input.Next<long>();
                pair.second = input.Next<long>();
                pairs.Add(new Pair<long, long>(pair));
            }
            pairs.Sort((a, b) => (int)(a.first - b.first));
            long num = 0;
            long cash = 0;
            for (int i = 0; i < N;i++){
                if(num+pairs[i].second>=M){
                    cash += (M - num) * pairs[i].first;
                    num = M;
                    break;
                }else{
                    num += pairs[i].second;
                    cash += pairs[i].second * pairs[i].first;
                }
            }
            Console.WriteLine(cash);
        }
    }


    public class Pair<S, T>
    {
        public Pair() { }
        public Pair(S f, T s)
        {
            first = f;
            second = s;
        }
        public Pair(Pair<S, T> p)
        {
            first = p.first;
            second = p.second;
        }
        public S first;
        public T second;
    }




    public class Input
    {
        int arg;
        int length;
        string s;
        string[] ss;
        char sep;
        public Input(char _sep = '\0')
        {
            sep = _sep;
        }
        public void ReadLine()
        {
            s = Console.ReadLine();
            if (sep != '\0') ss = s.Split(sep);
            else
            {
                ss = new string[1];
                ss[0] = s;
            }
            arg = 0;
            length = ss.Length;
        }
        public T Next<T>()
        {
            if (arg == length) throw new IndexOutOfRangeException();
            return (T)Convert.ChangeType(ss[arg++], typeof(T));
        }
        public void SetSeparater(char _sep)
        {
            sep = _sep;
        }
    }
}