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)

#pragma GCC optimize ("O3")
#include<bits/stdc++.h>
#include <queue>
#include <cstring>

#define pb push_back
#define p push

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define ll long long

#define getI(a) scanf("%d",&a)
#define getII(a, b) scanf("%d%d",&a,&b)
#define getIII(a, b, c) scanf("%d%d%d",&a,&b,&c)
#define getL(a) scanf("%lld",&a)
#define getLL(a, b) scanf("%lld%lld",&a,&b)
#define getLLL(a, b, c) scanf("%lld%lld%lld",&a,&b,&c)
#define BEND(c) (c).begin(),(c).end()
#define BEND2(c) (c).rbegin(),(c).rend()
#define fi first
#define se second
#define pi 3.1415926535897932384626
#define getS(a) scanf("%s",a)
#define pfb(a) printf("%.10lf\n",a);
#define pfi(a) printf("%d",a);
#define pfii(a, b) printf("%d%d",a,b);
#define pfl(a) printf("%lld",a);
#define pfll(a, b) printf("%lld%lld",a,b);
#define pfd(a) printf("%.12lf\n",a);
#define IOS ios_base::sync_with_stdio(false);cin.tie(0);
#define endl "\n"

#define SIZE  ( int)(1e9 + 7)
#define MAXN (ll) ((1e6+5))
#define K 25
#define  ALPHABET_SIZE  26
//#define N (int)(1e5+10)
#define sag (sol|1)
#define sol (root<<1)
#define ort (bas+son)/2
#define INF (ll )1e18
#define int ll
using namespace std;

typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<vector<int>> vvi;
typedef vector<vector<pair<int,int>>> vvpi;
typedef vector<long long> vl;
typedef vector<pair<int, int>> vpi;
typedef vector<pair<long long, long long>> vpl;
typedef pair<int, int> pii;
typedef pair <ll, ll> pll;
typedef vector<vi> graph ;



signed main ( ) {
    ll n ,k ;
    cin >> n >> k   ;
    vi vec ( n +1 );
    pair <ll , ll> mn (INF , 0);
    for (int i = 1; i <=n ; ++i) {
        cin >> vec[i];
        if (vec[i] < mn.fi) {
            mn.fi = vec[i];
            mn.se = i;
        }
    }
    ll res= 0 ;
    for (int j = mn.se; j <=n ; j +=(k-1) ) {
        while(vec[j]==mn.fi)j++;

        res++;
    }
    for (int l = mn.se;l >=1 ; l-=(k-1)) {
        while(vec[l]==mn.fi)l--;

        res++;
    }
    ll res2 = 0;
    for (int m = 1; m <=n ; m+=(k-1)) {
        while(vec[m] == mn.fi)m++;
        res2++;
    }
    cout << ceil( ((double)(n-1)) / (k-1));

}