Question:
A chemist is looking forward to create all possible chains of length n of atoms of Plutonium (Pu) and Lead (Pb), such that each chain that is formed cannot induce an atomic explosion. An explosion could occur if a chain contains at least two consecutive atoms of Plutonium.

For example, for n=4 some of the atomic chains that the scientist can create are:

Pb-Pb-Pu-Pb
Pb-Pb-Pb-Pb
Pu-Pb-Pb-Pu

But the following chain:

Pb-Pu-Pu-Pb

will lead to an explosion, because there are two consecutive Pu atoms.
How many "safe" chains of length n are there?

Input specification
You will be one positive integer n where 1 ≤ n ≤ 35.

Output specification
Show just one integer number: the number of safe chains.