public class Fibonacci5 {

    public static void main (String[] argv)
    {
        // Test case 1:
        int n = 5;
        int f = fibonacci (n);
        System.out.println ("f(" + n + ") = " + f);

        // Test case 2:
        n = 20;
        f = fibonacci (n);
        System.out.println ("f(" + n + ") = " + f);
    }

    static int fibonacci (int n)
    {
        // Base cases: same as in recursive version.
        if (n == 1) {
            return 0;
        }
        else if (n == 2) {
            return 1;
        }
        
        // Now we know n >= 3.

        // Start with first and second terms.
        int fPrev = 1;
        int fPrevPrev = 0;
        int f = -1;

        // A simple iteration takes us to the n-th term.
        for (int k=3; k<=n; k++) {
            f = fPrev + fPrevPrev;
            fPrevPrev = fPrev;
            fPrev = f;
        }
        
        return f;
    }

}