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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

// 最長N = 100,000 は
// 総和が(最長+1)*(最長/2)で10桁かなー
#define AN_LEN (10 + 1)  

// 要するに最小操作回数だけを求めるならば
// NとKだけで良い？
// 必ず1個ずれるみたいだから範囲-1の回数を
// 整数の数から引ける回数 = ans ということ？
int main(void)
{
    unsigned int  N, K;
    unsigned int  ans = 0;
    char An[AN_LEN];
    
    scanf("%d", &N);
    scanf("%d", &K);
    fgets(An, AN_LEN, stdin);

    while(1){
      if (N <= K){ 
        ans++;
        break;
      }else{
        N -= (K - 1);
        ans++;
      }
    }

    printf("%d", ans);
    return 0;
}