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 Ruby (2.3.3) (judge verdict: Accepted)

#coding: euc-jp
class Main
  def self.main
    n, m = Func.read1
    a, b = Func.read2(n)
    a, b = [a, b].mysort
    count = 0
    shop = 0
    price = 0
    while count < m
      if m - count > b[shop] then
        price += a[shop] * b[shop]
        count += b[shop]
      else
        price += a[shop] * (m - count)
        count += (m - count)
      end
      shop += 1
    end
    Func.write(price)
  end
end

module Function
  def read1
    return gets.chomp.split.map(&:to_i)
  end
  def read2(t)
    return t.times.map{gets.chomp.split.map(&:to_i)}.transpose
  end
  def write(ins)
    puts ins
  end
end

module Conv
  def mysort
    return self.transpose.sort{|a, b| a[0] <=> b[0]}.transpose
  end
end

class Func
  extend Function
end

class Array
  include Conv
end

Main.main