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)

import java.io.*;
import java.util.StringTokenizer;

class Main {

    public static void main(String[] args) throws IOException {
        init(System.in);
        int n = nextInt(), k = nextInt();
        int oneLeft = 0, oneRight = 0;
        int[] perm = new int[n + 1];
        for (int i = 1; i < perm.length; i++) {
            perm[i] = nextInt();
            if (perm[i] == 1) {
                oneLeft = i;
                oneRight = i;
            }
        }

        oneLeft = oneLeft - k + 1;
        if (oneLeft <= 0) oneRight = k;

        int steps = 1;
        while (oneLeft > 1) {
            steps++;
            oneLeft = oneLeft - k + 1;
        }
        while (oneRight < n) {
            steps++;
            oneRight = oneRight + k - 1;
        }

        System.out.println(steps);
    }

    //Input Reader
    private static BufferedReader reader;
    private static StringTokenizer tokenizer;

    private static void init(InputStream inputStream) {
        reader = new BufferedReader(new InputStreamReader(inputStream));
        tokenizer = new StringTokenizer("");
    }

    private static String next() throws IOException {
        String read;
        while (!tokenizer.hasMoreTokens()) {
            read = reader.readLine();
            if (read == null || read.equals(""))
                return "-1";
            tokenizer = new StringTokenizer(read);
        }

        return tokenizer.nextToken();
    }

    private static int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

//    private static long nextLong() throws IOException {
//        return Long.parseLong(next());
//    }
//
//    //    Get a whole line.
//    private static String line() throws IOException {
//        return reader.readLine();
//    }
//
//    private static double nextDouble() throws IOException {
//        return Double.parseDouble(next());
//    }
}