using System;

public class Program

{

    public static void Main()

    {

        //testing

        Console.WriteLine(climbStairs(3));

        Console.WriteLine(climbStairs(4));

        Console.WriteLine(climbStairs(5));

        Console.WriteLine(climbStairs(8));

    }

    public static int climbStairs(int numberOfSteps){

        if (numberOfSteps<1 || numberOfSteps>20){

            throw new ArgumentException("Number of steps cannot be less than 1 or greater than 20"); //exception to handle constraint 

        }

        //When trying to find a distinct number of ways to do something, a mathematical pattern is often useful. In this case,

        //for each increment in the number of stairs, the output of number of distinct ways to climb stairs start to resemble the fibonacci sequence

        //The following code is an implementation of the fibonacci sequence

        int i=0; 

        int k=1;

        for (int c=0;c<numberOfSteps;c++){

            int temp=k; //store k in temp variable to keep value of k

            k+=i;// increase k by i

            i=temp;//set i to previous value of k

        }

        return k;

    }   

}