Problem:
Score : 300 points

Problem Statement

There is a sequence of length N: A_1, A_2, ..., A_N. Initially, this sequence is a permutation of 1, 2, ..., N.

On this sequence, Snuke can perform the following operation:

Choose K consecutive elements in the sequence. Then, replace the value of each chosen element with the minimum value among the chosen elements.

Snuke would like to make all the elements in this sequence equal by repeating the operation above some number of times.
Find the minimum number of operations required.
It can be proved that, Under the constraints of this problem, this objective is always achievable.

Constraints

2 \leq K \leq N \leq 100000

A_1, A_2, ..., A_N is a permutation of 1, 2, ..., N.

Input

Input is given from Standard Input in the following format:

N K
A_1 A_2 ... A_N

Output

Print the minimum number of operations required.

Sample Input 1

4 3
2 3 1 4

Sample Output 1

2

One optimal strategy is as follows:

In the first operation, choose the first, second and third elements. The sequence A becomes 1, 1, 1, 4.

In the second operation, choose the second, third and fourth elements. The sequence A becomes 1, 1, 1, 1.

Sample Input 2

3 3
1 2 3

Sample Output 2

1

Sample Input 3

8 3
7 3 1 8 4 6 2 5

Sample Output 3

4

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Reflection.Emit;
using System.Text;
using static System.Console;
using static System.Math;

//using CS_Contest.Graph;
using CS_Contest.Loop;
using CS_Contest.Utils;
using static Nakov.IO.Cin;
using static CS_Contest.IO.IO;
using static CS_Contest.Utils.MyMath;


namespace CS_Contest {
    using bint=BigInteger;
	using Li = List<int>;
	using LLi = List<List<int>>;
	using Ll = List<long>;
	using ti3 = Tuple<int, int, int>;
	using ti2 = Tuple<int, int>;
    using tl3=Tuple<long,long,long>;
	internal class Program {
		private static void Main(string[] args) {
			var sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
			SetOut(sw);
			new Calc().Solve();
			Out.Flush();
		}

	    public class Calc
	    {
	        public void Solve() {
	            int N = NextInt(), K = NextInt();
	            var A = NextIntList();

	            ((int) Ceiling((double) (N - 1) / (K - 1))).WL();

	        }
	    }
    }
}
namespace Nakov.IO {
	using System;
	using System.Text;
	using System.Globalization;

	public static class Cin {
		public static string NextToken() {
			StringBuilder tokenChars = new StringBuilder();
			bool tokenFinished = false;
			bool skipWhiteSpaceMode = true;
			while (!tokenFinished) {
				int nextChar = Console.Read();
				if (nextChar == -1) {
					tokenFinished = true;
				} else {
					char ch = (char)nextChar;
					if (char.IsWhiteSpace(ch)) {
						if (!skipWhiteSpaceMode) {
							tokenFinished = true;
							if (ch == '\r' && (Environment.NewLine == "\r\n")) {
								Console.Read();
							}
						}
					} else {
						skipWhiteSpaceMode = false;
						tokenChars.Append(ch);
					}
				}
			}

			string token = tokenChars.ToString();
			return token;
		}

		public static int NextInt() {
			string token = Cin.NextToken();
			return int.Parse(token);
		}
		public static long NextLong() {
			string token = Cin.NextToken();
			return long.Parse(token);
		}
		public static double NextDouble(bool acceptAnyDecimalSeparator = true) {
			string token = Cin.NextToken();
			if (acceptAnyDecimalSeparator) {
				token = token.Replace(',', '.');
				double result = double.Parse(token, CultureInfo.InvariantCulture);
				return result;
			} else {
				double result = double.Parse(token);
				return result;
			}
		}
		public static decimal NextDecimal(bool acceptAnyDecimalSeparator = true) {
			string token = Cin.NextToken();
			if (acceptAnyDecimalSeparator) {
				token = token.Replace(',', '.');
				decimal result = decimal.Parse(token, CultureInfo.InvariantCulture);
				return result;
			} else {
				decimal result = decimal.Parse(token);
				return result;
			}
		}

	}
}

namespace CS_Contest.Loop {
	[DebuggerStepThrough]
	public static class Loop {
		public static void REP(this int n, Action<int> act) {
			for (var i = 0; i < n; i++) {
				act(i);
			}
		}

		public static void ForeachWith<T>(this IEnumerable<T> ie, Action<int, T> act) {
			var i = 0;
			foreach (var item in ie) {
				act(i, item);
				i++;
			}
		}

		public static void Foreach<T>(this IEnumerable<T> ie, Action<T> act) {
			foreach (var item in ie) {
				act(item);
			}
		}

	}

	public class Generate
	{
	    public static IEnumerable<int> Seq(int e) => Seq(0, e, 1);
		public static IEnumerable<int> Seq(int s, int e) => Seq(s, e, 1);
		public static IEnumerable<int> Seq(int s, int e, int a) {
			while (s != e) {
				yield return s;
				s += a;
			}
		}
		public static List<T> Repeat<T>(Func<int, T> result, int range) =>
			Enumerable.Range(0, range).Select(result).ToList();
	}
}

namespace CS_Contest.IO {
	using Li = List<int>;
	using Ll = List<long>;

	public static class IO {
		public static void WL(this object obj) => WriteLine(obj);
		public static void WL(this string obj) => WriteLine(obj);
		public static void WL<T>(this IEnumerable<T> list) => list.ToList().ForEach(x => x.WL());

		public static Li NextIntList() => ReadLine().Split().Select(int.Parse).ToList();
		public static Li NextIntList(int n) => Enumerable.Repeat(0, n).Select(x => ReadLine()).Select(int.Parse).ToList();
		public static Ll NextLongList() => ReadLine().Split().Select(long.Parse).ToList();

		public static T Tee<T>(this T t, Func<T, string> formatter = null) {
			if (formatter == null) formatter = arg => arg.ToString();
			formatter(t).WL();
			return t;
		}
		public static void JoinWL<T>(this IEnumerable<T> @this, string sp = " ") => @this.StringJoin(sp).WL();
		public static void W(this object @this) => Write(@this);

	    public static T[,] GetBox<T>(int h, int w, Func<int, int, T> getFunc) {
	        var rt = new T[h, w];
	        for (int i = 0; i < h; i++) {
	            for (int j = 0; j < w; j++) {
	                rt[i, j] = getFunc(i, j);
	            }
	        }

	        return rt;
	    }

	}


}

namespace CS_Contest.Utils {
	using Li = List<int>;
	using Ll = List<long>;
    using bint = BigInteger;

    [DebuggerStepThrough]
    public static class Utils {
        public static bool AnyOf<T>(this T @this, params T[] these) where T : IComparable {
            return these.Contains(@this);
        }

        public static bool Within(int x, int y, int lx, int ly) => !(x < 0 || x >= lx || y < 0 || y >= ly);

        public static void Add<T1, T2>(this List<Tuple<T1, T2>> list, T1 t1, T2 t2) =>
            list.Add(new Tuple<T1, T2>(t1, t2));

        public static string StringJoin<T>(this IEnumerable<T> l, string separator = "") => string.Join(separator, l);

        public static Queue<T> ToQueue<T>(this IEnumerable<T> iEnumerable) {
            var rt = new Queue<T>();
            foreach (var item in iEnumerable) {
                rt.Enqueue(item);
            }

            return rt;
        }

        public static void Swap<T>(ref T x, ref T y) {
            var tmp = x;
            x = y;
            y = tmp;
        }

        public static List<Tuple<TKey, TValue>> ToTupleList<TKey, TValue>(this Map<TKey, TValue> @this) =>
            @this.Select(x => Tuple.Create(x.Key, x.Value)).ToList();


        public static Map<TKey, int> CountUp<TKey>(this IEnumerable<TKey> l) {
            var dic = new Map<TKey, int>();
            foreach (var item in l) {
                dic[item]++;
            }

            return dic;
        }

        public static int Count<T>(this IEnumerable<T> l, T target) => l.Count(x => x.Equals(target));

        public static IEnumerable<T> SkipAt<T>(this IEnumerable<T> @this, int at) {
            var enumerable = @this as T[] ?? @this.ToArray();
            for (var i = 0; i < enumerable.Count(); i++) {
                if (i == at) continue;
                yield return enumerable.ElementAt(i);
            }
        }

        public static int LowerBound<T>(this List<T> @this, T x) where T : IComparable {
            int lb = -1, ub = @this.Count;
            while (ub - lb > 1) {
                int mid = (ub + lb) >> 1;
                if (@this[mid].CompareTo(x) >= 0) ub = mid;
                else lb = mid;
            }

            return ub;
        }

        public static int UpperBound<T>(this List<T> @this, T x) where T : IComparable {
            int lb = -1, ub = @this.Count;
            while (ub - lb > 1) {
                int mid = (ub + lb) >> 1;
                if (@this[mid].CompareTo(x) > 0) ub = mid;
                else lb = mid;
            }

            return ub;
        }

        public static List<int> Imos(this IEnumerable<int> @this) {
            var rt = @this.ToList();
            for (var i = 0; i < rt.Count-1; i++) rt[i + 1] += rt[i];
            return rt;
        }

    }



    public class Map<TKey, TValue> : Dictionary<TKey, TValue> {
		public Map() : base() { }
		public Map(int capacity) : base(capacity) { }

		public new TValue this[TKey index] {
			get {
				TValue v;
				return this.TryGetValue(index, out v) ? v : base[index] = default(TValue);
			}
			set { base[index] = value; }
		}
	}

	public static class MyMath {
		
		public static T EMin<T>(params T[] a) where T : IComparable<T> => a.Min();
		public static T EMax<T>(params T[] a) where T : IComparable<T> => a.Max();

	}


}