program ABC_079_B_LucasNumber
   implicit none
   integer N, i
   integer(8) L(0: 86)
   read(*, *) N
   L(0) = 2
   L(1) = 1
   do i = 2, N
       L(i) = L(i-1) + L(i-2)
   end do
   write(*, *) L(N)
end program ABC_079_B_LucasNumber