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)

n, m = gets.chomp.split(" ").map(&:to_i)
a_ary = []
n.times do
  a_ary.push(gets.chomp.split(" ").map(&:to_i))
end
sorted_a_ary = a_ary.sort {|a,b| a[0] <=> b[0]}
drink_sum = 0
money_sum = 0
last_shop = 0
sorted_a_ary.each_with_index do |sorted_a, index|
  drink_sum += sorted_a[1]
  if drink_sum >= m
    last_shop = index
    break
  end
end
drink_sum = 0
if last_shop > 0
  sorted_a_ary[0..last_shop - 1].each_with_index do |sorted_a, index|
    drink_sum += sorted_a[1]
    money_sum += sorted_a[0] * sorted_a[1]
  end
end
money_sum += (m - drink_sum) * (sorted_a_ary[last_shop][0])
puts money_sum