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: Accepted)

import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

	void run() {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();
		int K = sc.nextInt();

		int bunsi=(N-1);
		int bunbo=K-1;

		int ans=(bunsi+bunbo-1)/bunbo;

		System.out.println(ans);
	}


	//以下テンプレート

	public static int[] extgcd(int a, int b) {

		int x0 = 1;
		int x1 = 0;

		int y0 = 0;
		int y1 = 1;

		while (b != 0) {
			int q = a / b;
			int r = a % b;

			int x2 = x0 - q * x1;
			int y2 = y0 - q * y1;

			a = b;
			b = r;

			x0 = x1;
			x1 = x2;

			y0 = y1;
			y1 = y2;
		}

		return new int[] { a, x0, y0 };

	}

	static int gcd(int a, int b) {

		if (b == 0)
			return a;

		if (a < b) {
			int t = a;
			a = b;
			b = t;
		}

		return gcd(b, a % b);

	}

	static int lcm(int a, int b) {
		return a * b / gcd(a, b);
	}

	static void swap(int[] a) {
		int t;

		t = a[0];
		a[0] = a[1];
		a[1] = t;

		return;
	}

	static <T> void output(List<T> list) {

		for (int i = 0; i < list.size(); i++) {
			System.out.print(list.get(i));

			if (i != list.size() - 1) {
				System.out.print(" ");
			} else {
				nl();
			}
		}

	}

	static void output(String[][] str) {

		for (int i = 0; i < str.length; i++) {
			for (int j = 0; j < str[i].length; j++) {
				print(str[i][j]);
			}

			nl();
		}

	}

	static void output(boolean flg, String Yes, String No) {

		if (flg) {
			pln(Yes);
		} else {
			pln(No);
		}

	}

	static void output(String[][] str, int digit) {

		String dig = "%" + String.valueOf(digit) + "s";

		for (int i = 0; i < str.length; i++) {
			for (int j = 0; j < str[i].length; j++) {
				System.out.printf(dig, str[i][j]);
			}
			nl();
		}

	}

	static void pln(String str) {
		System.out.println(str);
	}

	static void pln(int x) {
		System.out.println(x);
	}

	static void print(String str) {
		System.out.print(str);
	}

	static void print(int x) {
		System.out.print(x);
	}

	static void print(String str, int times) {
		for (int i = 0; i < times; i++) {
			print(str);
		}
	}

	static void print(int x, int times) {
		for (int i = 0; i < times; i++) {
			print(x);
		}
	}

	static void nl() {
		System.out.println();
	}

}