program main
  implicit none
  integer::b(3),next
  real*8::r,d
  read *,b
  ! b(3) = r*b(2)+d
  ! b(2) = r*b(1)+d
  ! b3-b2 = r(b2-b1)
  r = real(b(3)-b(2))/real(b(2)-b(1))
  d = b(3)-r*b(2)
  next = NINT(r*b(3)+d)
!  print *, b,r,d
  print'(i0)',next
end program main