Competitions

# Dynamic Programming - Linear

Long before Sharik found a clever book, lost Pechkin when he first began his experiments on sawing chess boards, even when on the chessboard white fields were white, and black - the black, he asked one of his first Matroskin brainteasers.

"_How many different sequences of length n can be formed from cells of sawn chessboards, if none of the sequences of any three white field should not go straight._"

Matroskin have not decided yet this puzzle, so your task is to help him.

#### Input

The length n (n64) of the sequence.

#### Output

Print the number of such sequences.

Time limit 1 second
Memory limit 128 MiB
Input example #1
1

Output example #1
2

Input example #2
2

Output example #2
4

Input example #3
3

Output example #3
7