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)

raw1 = gets.split(" ").map{|nn|nn.to_i}
n = raw1[0]
request_amount = raw1[1]

class Shop
  attr_accessor :price,:amount
  def initialize(a,b)
    self.price = a;
    self.amount = b;
  end

  def <=> (other)
    return self.price - other.price
  end
end

shops = []
n.times do
  raw = gets.split(" ").map{|nn|nn.to_i}
  shops << Shop.new(raw[0],raw[1])
end

shops.sort!{|nn,mm| nn <=> mm};
money = 0
bought = 0
shops.each{|elem|
  while(bought < request_amount)
    money += elem.price
    bought += 1
    elem.amount -= 1
    break if elem.amount == 0
  end
}

puts money.to_s