program main
  implicit none
  integer*8::b(3),next
  real*8::r,d
  read *,b
  r = real(b(3)-b(2),kind=8)/real(b(2)-b(1),kind=8)
  d = b(3)-r*b(2)
  next = NINT(r*b(3)+d,kind=8)
!  print *, b,r,d,r*b(3)+d
  print'(i0)',next
end program main