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.Comparator;
import java.util.Scanner;

/* static int java.util.Arrays.binarySearch(int[] a, int key); */


public class Main {
    Scanner sc = new Scanner(System.in);
    int n, k;
    public void run(){
        n = sc.nextInt();
        k = sc.nextInt();
        solve();
    }
    public void solve(){
        int i = 0;
        int ans = 0;

        while(i < n){
            int flag = 0;
            ans++;
            for(int j = 0; j < k-1; j++){
                if(i < n) {
                    int a = sc.nextInt();
                    i++;

                    if (a == 1) {
                        flag = 1;
                    }
                }
                else{
                    break;
                }
            }
            if(flag == 1 && i < n){
                sc.nextInt();
                i++;
            }
        }
        System.out.println(ans);
    }

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

    static class Pair {
        int first,second;
        public Pair(int a,int b) {
            this.first = a;
            this.second = b;
        }
    }
    static class Edge{
        int to, cost;
        public Edge(int a, int b){
            this.to = a;
            this.cost = b;
        }
    }
    class MyComparator implements Comparator {
        @Override
        public int compare (Object arg0, Object arg1) {
            int x = ((Pair)arg0).first;
            int y = ((Pair)arg1).first;

            if (x < y) {
                return 1;
            } else if (x > y) {
                return -1;
            } else{
                return 0;
            }
        }
    }
    static class UnionFind{
        int par[];
        int rank[];

        //n要素で初期化
        public UnionFind(int maxn){
            par = new int[maxn];
            rank = new int[maxn];

            for(int i = 0; i < maxn; i++){
                par[i] = i;
                rank[i] = 0;
            }
        }

        //木の根を求める
        int find(int x){
            if(par[x] == x){
                return x;
            }
            else {
                return par[x] = find(par[x]);
            }
        }

        // xとyの属する集合を併合
        void unite (int x, int y){
            x = find(x);
            y = find(y);
            if(x == y){
                return;
            }

            if(rank[x] < rank[y]){
                par[x] = y;
            }
            else{
                par[y] = x;
                if(rank[x] == rank[y]) rank[x]++;
            }
        }

        // xとyが同じ集合に属するか否か
        boolean same(int x, int y){
            return find(x) == find(y);
        }
    }

}