IprKvx - Online C# Compiler & Debugging Tool

using System;

public class Test
{
	static void DoNothingWith(int n) {}
	static int FibR(int n) {
    var result = (n < 2)? n : Fib(n - 1) + Fib(n - 2);
    DoNothingWith(result);
    return result;
}

	static int Fib(int x) {
    if (x == 0) return 0;

    int prev = 0;
    int next = 1;
    for (int i = 1; i < x; i++)
    {
        int sum = prev + next;
        prev = next;
        next = sum;
    }
    return next;
}

	public static void Main()
	{
		Console.Write(FibR(2000000000));
	}
}