// The Fibonacci sequence is defined as a[0]=0, a[1]=1, a[k]=a[k-1] + a[k-2] for k>1.

// Write a program that inputs nonnegative k and prints a[k].

using System;

using System.Diagnostics;

public class Program

{

    public static int InputInt(string prompt)

    {

        Console.Write("Enter " + prompt + ": ");

        return Convert.ToInt32(Console.ReadLine());

    }

    public static void Main(string[] args)

    {

        int n = InputInt("n");

        Trace.Assert(n >= 0);

        if(n==0)

        {

            Console.WriteLine("fib(0) = 0");

            return;

        }

        int i = 1;

        int fib_last = 1;

        int fib_prev = 0;

        // invariant: fib_last=fib(i) and fib_prev=fib(i-1) and 1<=i<=n

        while (i != n)

        {

            int fib_next = fib_last + fib_prev;

            // fib_next = fib(i+1) and i<n

            i++;

            // fib_next= fib(i) and fib_last = fib(i-1) and fib_prev = fib(i-2) and i<=n

            fib_prev = fib_last;

            fib_last = fib_next;

            // fib_last = fib(i) and fib_prev = fib(i-1) and i<=n

        }

        Console.WriteLine("fib(" + n + ") = " + fib_last);

    }

}

​

​