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++14 (GCC 5.4.1) (judge verdict: Accepted)

/*
for (int i = 0; i < N; ++i){}

// 各桁の和を計算する関数
int findSumOfDigits(int n) {
  int sum = 0;
  while (n > 0) { // n が 0 になるまで
    sum += n % 10;
    n /= 10;
  }
  return sum;
}

sort(a, a + N, greater<int>());  // a[0:N] を大きい順にソート

int num[110] = {0};  // バケット
for (int i = 0; i < N; ++i) {
    num[d[i]]++;  // d[i] が 1 個増える
}

map<string, int> mp; // 連想配列 map<キー型, 値型> オブジェクト名
for (int i = 0; i < N; ++i) {
    auto itr = mp.find(s[i]);        // s[i] が設定されているか？
    if(itr != mp.end() ) {
         mp[s[i]] += 1;
    }
    else {
         mp[s[i]] += 1 ;
    }
}


*/
typedef long long ll;
#include <iostream>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <algorithm>

using namespace std;

int findSumOfDigits(int n) {
  int sum = 0;
  while (n > 0) { // n が 0 になるまで
    sum += n % 10;
    n /= 10;
  }
  return sum;
}


int main() {
    // 入力
    ll N, K;
    cin >> N >> K;
    ll A[N];
    for (int i = 0; i < N; ++i) cin >> A[i];
    
    ll minA=A[0];
    for (int i = 0; i < N; ++i){
        if (A[i]<minA) minA = A[i];
    }
    //cout <<"min A " << minA << endl;
    
    ll count_not_min_A = 0;
    
    for (int i = 0; i < N; ++i){
        if (A[i]!=minA) ++count_not_min_A;
    }
    //cout << "not min A " << count_not_min_A << endl;
    
    float ans;
    ans = float(count_not_min_A) / (K - 1);
    if (int(ans)<ans) ans = int(ans)+1;
    cout << ans << endl;
    //cout << int(ans) << endl;
}