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 Fortran (gfortran v4.8.4) (judge verdict: Wrong Answer)

program main
	implicit none
    integer :: n, m
    integer, allocatable :: a(:), b(:)
    integer :: i, j, tmp1, tmp2
    integer :: tenpo, kakaku, kosu, amari
    read(*, *) n, m
    allocate(a(n)); allocate(b(n))
    do i = 1, n
    	read(*, *) a(i), b(i)
    enddo
    do i = 1, n
    	do j = i+1, n
        	if (a(i).lt.a(j)) then
            	tmp1 = a(i)
                a(i) = a(j)
                a(j) = tmp1
                tmp2 = b(i)
                b(i) = b(j)
                b(j) = tmp2
            end if
		enddo
	enddo
    kosu = 0
    tenpo = 0
    do i = 1, n
		kosu = kosu + b(i)
        tenpo = tenpo + 1
        if (kosu.ge.m) then
        	amari = kosu - m
            exit
        end if
    enddo
    do i = 1, tenpo-1
    	kakaku = kakaku + a(i)*b(i)
    enddo
    kakaku = kakaku + a(tenpo)*(b(tenpo)-amari)
    write(*, *) kakaku
stop
end program main