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;

namespace ABC121 {
  class ProblemC {
    static void Main(string[] args) {
      string[] str1 = System.Console.ReadLine().Split(' '), str2;
      int n = int.Parse(str1[0]), m = int.Parse(str1[1]);
      Tuple<int, int>[] ab = new Tuple<int, int>[n];
      for (int i = 0; i < n; i++) {
        str2 = System.Console.ReadLine().Split(' ');
        ab[i] = Tuple.Create<int, int>(int.Parse(str2[0]), int.Parse(str2[1]));
      }

      long cost = 0L;
      int rest = m, j = 0;
      Array.Sort(ab);
      while (rest > 0) {
        if (rest > ab[j].Item2) {
          cost += (long)ab[j].Item1 * (long)ab[j].Item2;
          rest -= ab[j].Item2;
        } else {
          cost += (long)ab[j].Item1 * (long)rest;
          rest = 0;
        }
        j++;
      }
      System.Console.WriteLine(cost);
    }
  }
}