using System.Text;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;

namespace ConsoleApp1
{
    public class A
    {
        void SolveTest(TextReader reader, TextWriter writer, int testNumber)
        {
            var line1 = reader.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
            int W = line1[0];
            int R = line1[1];
            int N = line1[2];

            var lights = reader.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
            if (lights[0] - R > 0)
            {
                writer.WriteLine($"Case #{testNumber}: IMPOSSIBLE");
                return;
            }
         
            int count1 = 0;
            for (int i = 0; i < N; ++i)
            {
                count1++;
                if (lights[i] + R >= W)
                    break;
                if (i > 0 && lights[i - 1] + R < lights[i] - R)
                {
                    writer.WriteLine($"Case #{testNumber}: IMPOSSIBLE");
                    return;
                }
            }
            if (lights[N-1] + R < W)
            {
                writer.WriteLine($"Case #{testNumber}: IMPOSSIBLE");
                return;
            }
            int count2 = 0;
            for (int i = N-1; i >= 0; --i)
            {
                count2++;
                if (lights[i] - R <= 0) break;
              
            }
            
            writer.WriteLine($"Case #{testNumber}: {System.Math.Min(count1, count2)}");
        }

        void SolveAll(TextReader reader, TextWriter writer)
        {
            int testCount = int.Parse(reader.ReadLine());
            for (int i = 0; i < testCount; ++i)
            {
                SolveTest(reader, writer, i + 1);
            }
        }

        private static readonly string inputFileName = "input.txt";
        private static readonly string outputFileName = "output.txt";

        public static void Main(string[] args)
        {
            A a = new A();
            if (args.Contains("file"))
            {
                using (var reader = new StreamReader(new FileStream(inputFileName, FileMode.Open)))
                {
                    using (var writer = new StreamWriter(outputFileName))
                    {
                        a.SolveAll(reader, writer);
                    }
                }
            }
            else
            {
                a.SolveAll(System.Console.In, System.Console.Out);
            }
        }
    }
}