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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

class Main {
  public static void main(String[] args) {
      java.util.Scanner sc = new java.util.Scanner(System.in);
      int N=sc.nextInt();
      int K=sc.nextInt();
      int A[]=new int [N];
      int count=0;
      int serch1=0;
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
          //A[]の何番目が1なのかを探す
      if(A[i]==1){
         serch1=i+1;
      }
    }
    
      count+=( (serch1-1)/(K-1) );
      if(( (serch1-1)%(K-1) )==1){
        count++;
      }
      count+=( (N-serch1)/(K-1) );
      if(( (N-serch1)%(K-1) )==1){
        count++;
      }
      println(count);
  }
  /* 以下、標準出力用関数 */
  static public void print(byte what) { System.out.print(what); System.out.flush(); }
  static public void print(boolean what) { System.out.print(what); System.out.flush(); }
  static public void print(char what) { System.out.print(what); System.out.flush(); }
  static public void print(int what) { System.out.print(what); System.out.flush(); }
  static public void print(long what) { System.out.print(what); System.out.flush(); }
  static public void print(float what) { System.out.print(what); System.out.flush(); }
  static public void print(double what) { System.out.print(what); System.out.flush(); }
  static public void print(String what) { System.out.print(what); System.out.flush(); }
  static public void print(Object... variables) { StringBuilder sb = new StringBuilder(); for (Object o : variables) { if (sb.length() != 0) { sb.append(" "); } if (o == null) { sb.append("null"); } else { sb.append(o.toString()); } } System.out.print(sb.toString()); }
  static public void println() { System.out.println(); }
  static public void println(byte what) { System.out.println(what); System.out.flush(); }
  static public void println(boolean what) { System.out.println(what); System.out.flush(); }
  static public void println(char what) { System.out.println(what); System.out.flush(); }
  static public void println(int what) { System.out.println(what); System.out.flush(); }
  static public void println(long what) { System.out.println(what); System.out.flush(); }
  static public void println(float what) { System.out.println(what); System.out.flush(); }
  static public void println(double what) { System.out.println(what); System.out.flush(); }
  static public void println(String what) { System.out.println(what); System.out.flush(); }
  static public void println(Object... variables) { print(variables); println(); }
  static public void println(Object what) { if (what == null) { System.out.println("null"); } else if (what.getClass().isArray()) { printArray(what); } else { System.out.println(what.toString()); System.out.flush(); } }
  static public void printArray(Object what) { if (what == null) { System.out.println("null"); } else { String name = what.getClass().getName(); if (name.charAt(0) == '[') { switch (name.charAt(1)) { case '[': System.out.println(what); break; case 'L': Object poo[] = (Object[]) what; for (int i = 0; i < poo.length; i++) { if (poo[i] instanceof String) { System.out.println("[" + i + "] \"" + poo[i] + "\""); } else { System.out.println("[" + i + "] " + poo[i]); } } break; case 'Z': boolean zz[] = (boolean[]) what; for (int i = 0; i < zz.length; i++) { System.out.println("[" + i + "] " + zz[i]); } break; case 'B': byte bb[] = (byte[]) what; for (int i = 0; i < bb.length; i++) { System.out.println("[" + i + "] " + bb[i]); } break; case 'C': char cc[] = (char[]) what; for (int i = 0; i < cc.length; i++) { System.out.println("[" + i + "] '" + cc[i] + "'"); } break; case 'I': int ii[] = (int[]) what; for (int i = 0; i < ii.length; i++) { System.out.println("[" + i + "] " + ii[i]); } break; case 'J': long jj[] = (long[]) what; for (int i = 0; i < jj.length; i++) { System.out.println("[" + i + "] " + jj[i]); } break; case 'F': float ff[] = (float[]) what; for (int i = 0; i < ff.length; i++) { System.out.println("[" + i + "] " + ff[i]); } break; case 'D': double dd[] = (double[]) what; for (int i = 0; i < dd.length; i++) { System.out.println("[" + i + "] " + dd[i]); } break; default: System.out.println(what); } } else { System.out.println(what); } } System.out.flush(); }}