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)

#include "bits/stdc++.h"
using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long long> vll;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<pair<long long,long long> > vpll;
typedef vector<vector<pair<ll,ll> > > vvpll;
typedef vector<bool> vb;
typedef vector<vb> vvb;

#define FIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define all(x) x.begin() , x.end()
#define pb push_back
#define bs binary_search
#define lb lower_bound
#define ub upper_bound
#define mp make_pair
#define F first
#define S second
#define left (node << 1)
#define right (node << 1) + 1
#define pii pair<int,int> 
#define mii map<int,int>
#define vi vector<int>
#define umii unordered_map<int,int>
#define mod1 998244353
#define mod2 1000000007
#define trace1(x)                cerr<<#x<<": "<<x<<endl
#define trace2(x, y)             cerr<<#x<<": "<<x<<" | "<<#y<<": "<<y<<endl
#define trace3(x, y, z)          cerr<<#x<<": " <<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl
#define FILL(name,val) memset(name , val , sizeof(name))
#define repo(i,o,e) for(int i = o ; i < e ; i++)
#define rep(n) for(int i = 0 ; i < n ; i++)
const ll sz = 2e5+5;
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};

const ll inf = 1000LL * 1000LL * 1000LL * 1000LL * 1000LL * 1000LL;

void solve(){
    int n , k , x;
    cin >> n >> k;
    rep(n){
        cin >> x;
    }
    int f = k , cnt = 1;
    while(f + k - 1 <= n){
        f += k - 1;
        cnt++;
    }
    if(f < n)cnt++;
    cout << cnt << " ";
    return;
}



int main(){
    FIO;
    int t = 1;
    // cin >> t;
    while(t--){
        solve();
    }
    return 0;
}