using System;
    using System.Linq;
    public class VJ
    {
        static int Main()
        {
            process();
            Console.ReadLine();
            return 0;
        }
        static void process()
        {
            string str;
            int[] arr;
            int m,sum;
            while((str=Console.ReadLine())!=null)
            {
                arr=Array.ConvertAll<string , int>(str.Split(‘ ‘) , r => int.Parse(r));
                m=Convert.ToInt32((str=Console.ReadLine())[0].ToString());
                sum=(from int r in arr select r).Sum()-arr[0];
                if(m==0) Console.WriteLine("{0:0.00}" , (double)sum/(double)arr[0]);
                else if(m==arr[0]) Console.WriteLine("inf");
                else Console.WriteLine("{0:0.00}" , (double)sum/(double)(arr[0]-m));
            }
        }
    }