program atcoder
implicit none
integer(16) :: n,m,i,ans,num,t
integer(16), allocatable :: a(:),b(:),turn(:)

read(*,*) n,m
allocate(a(n),b(n),turn(n))

do i = 1,n
	read(*,*) a(i), b(i)
end do

if(n == 1)then
	write(*,'(i0)') m*a(n)
    stop
end if

call hs2(n,a,turn)

ans = 0
num = 0
do i = 1, n
	t = b(turn(i))
	num = num + t
    ans = ans + a(i)*t
	if(num >= m)then
    	ans = ans - (num-m)*a(i)
        write(*,'(i0)') ans
        stop
    end if
end do

contains
subroutine hs2(n,array,turn)
  implicit none
  integer(16),intent(in)::n
  integer(16),intent(out)::turn(1:n)
  integer(16),intent(inout)::array(1:n)
  
  integer(16)::i,k,j,l,m
  integer(16)::t
  
  if(n.le.0)then
     write(6,*)"Error, at heapsort"; stop
  endif
  if(n.eq.1)return

  do i=1,N
     turn(i)=i
  enddo

  l=n/2+1
  k=n
  do while(k.ne.1)
     if(l.gt.1)then
        l=l-1
        t=array(l)
        m=turn(l)
     else
        t=array(k)
        m=turn(k)
        array(k)=array(1)
        turn(k)=turn(1)
        k=k-1
        if(k.eq.1) then
           array(1)=t
           turn(1)=m
           exit
        endif
     endif
     i=l
     j=l+l
     do while(j.le.k)
        if(j.lt.k)then
           if(array(j).lt.array(j+1))j=j+1
        endif
        if (t.lt.array(j))then
           array(i)=array(j)
           turn(i)=turn(j)
           i=j
           j=j+j
        else
           j=k+1
        endif
     enddo
     array(i)=t
     turn(i)=m
  enddo

  return
end subroutine hs2

end program atcoder