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)

def input_n
  gets.to_i               # 単一整数
end

def input_s
  gets.chomp              # 文字列。chompを付けないと改行文字がついてくる
end

def input_a
  gets.split.map(&:to_i)  # スペースで区切られた複数の整数
end

def input_o(num)
  num.times.map { gets.to_i }  # 縦に並んだ複数の整数。たまにある
end

def output_f(num)
  puts sprintf("%.12f", num)
end

n, m = input_a
a = n.times.map { gets.split.map(&:to_i) }
a.sort_by! { |a| a[0] }
sum = 0
cnt = 0
a.each do |aaa, bbb|
  if m < cnt + bbb
    sum += aaa * (m - cnt)
    break
  else
    sum += aaa * bbb
    cnt += bbb
  end
end
puts sum