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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
class pro
{
  static void Main()
  {
    int n=int.Parse(Console.ReadLine());
    Console.WriteLine(Luca(n));
  }
  static long Luca(int n)
  {
    long prev1=2;
    long prev0=1;
    long now=0;
    if(n==0)
      return 2;
    else if(n==1)
      return 1;
    else
    {
      for(int i=2;i<=n;i++)
      {
        var result=prev0+prev1;
        prev1=prev0;
        prev0=result;
        now=result;
      }
      return now;
    }
  }
}