using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {

            string[] tokens = Console.ReadLine().Split();
            ulong a = ulong.Parse(tokens[0]);
            ulong b = ulong.Parse(tokens[1]);
            ulong c = ulong.Parse(tokens[2]);

            ulong count = 0;
            for (ulong i = 0; i < b; i++)
            {
                if (c == 1)
                {
                    Console.WriteLine("-1");
                    break;
                }
                if (a < b)
                {
                    a *= c;
                    count++;
                }
                if (a >= b)
                {
                    Console.WriteLine(count);
                    break;
                }

            }
        }
    }
}