using System;

namespace _3 {
    class Program {

        static int Fibb(int[] arr, int elem) {
            int l = 0, r = arr.Length - 1;
            while (r - l > 1) {
                int a = 1, b = 2, c;
                while (l + b - 1 <= r && arr[l + b - 1] <= elem) {
                    c = a + b;
                    a = b;
                    b = c;
                    //Console.WriteLine($"a = {a}, b = {b}");
                }
                //Console.WriteLine("---");
                l = Math.Max(l + a - 1, 0);
                r = Math.Min(l + b - 1, arr.Length - 1);
            }
            //Console.WriteLine($"l = {l}, r = {r}");
            int ret = -1;
            if (arr[l] == elem)
                ret = l;
            else if (arr[r] == elem)
                ret = r;
            return ret;
        }

        static void Main(string[] args) {
            int[] arr = {1, 5, 10, 11, 12, 15, 17};
            Console.WriteLine(Fibb(arr, 14));
        }
    }
}