Problem:
Score : 200 points

Problem Statement

It is November 18 now in Japan. By the way, 11 and 18 are adjacent Lucas numbers.

You are given an integer N. Find the N-th Lucas number.

Here, the i-th Lucas number L_i is defined as follows:

L_0=2

L_1=1

L_i=L_{i-1}+L_{i-2} (i≥2)

Constraints

1≤N≤86

It is guaranteed that the answer is less than 10^{18}.

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the N-th Lucas number.

Sample Input 1

5

Sample Output 1

11

L_0=2

L_1=1

L_2=L_0+L_1=3

L_3=L_1+L_2=4

L_4=L_2+L_3=7

L_5=L_3+L_4=11

Thus, the 5-th Lucas number is 11.

Sample Input 2

86

Sample Output 2

939587134549734843

Solution in Ruby (2.3.3) (judge verdict: Accepted)

# 今、日本は 11 月 18 日ですが、11 と 18 は隣り合うリュカ数です。
# 整数 N が与えられるので、N 番目のリュカ数を求めてください。
# ただし、リュカ数は i 番目のリュカ数を Li とすると、
# L0=2
# L1=1
# Li=Li−1+Li−2(i≧2)
# と定義される数とします。

n = gets.to_i

l = [2, 1]

(n - 1).times do |i|
  l << (l[i] + l[i+1])
end

puts l.last