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 9.2.1) (judge verdict: Accepted)

/*
URL_HERE
*/
/*

*/
#ifdef _WIN32
#pragma warning(disable:4996)
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <algorithm>
#include <math.h>
#include <unordered_map>
using namespace std;

FILE* _fin = stdin;
FILE* _fout = stdout;
#define PI 3.141592653589793238462643383279502884197169399375105820974
#define ten5p1 100001
#define ten6p1 1000001
#define ten8p1 100000001
#define ten9p1 1000000001
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define rep(var,n) for(int var=0;var<n;++var)
#define repi(n) rep(i,n)
#define repj(n) rep(j,n)
#define repi1(n) for(int i=1;i<n;++i)
#define repj1(n) for(int j=1;j<n;++j)
int _min(int a, int b) { return a <= b ? a : b; }
int _min(ll a, ll b) { return a <= b ? a : b; }
int _max(int a, int b) { return a >= b ? a : b; }
int _max(ll a, ll b) { return a >= b ? a : b; }
void zero(int* data, int n) { memset(data, 0, sizeof(int) * n); }
void zero(ll* data, int n) { memset(data, 0, sizeof(ll) * n); }
void zero(char* data, int n) { memset(data, 0, sizeof(char) * n); }
char readc() { char var; fscanf(_fin, "%c", &var); return var; }
int readi() { int var; fscanf(_fin, "%d", &var); return var; }
double readlf() { double var; fscanf(_fin, "%lf", &var); return var; }
ll readll() { ll var; fscanf(_fin, "%lld", &var); return var; }
void repread(int* data, int n) { repi(n) data[i] = readi(); }
void repread(ll* data, int n) { repi(n) data[i] = readll(); }
int reads(char* str, int maxsize)
{
	for (;;) { if (fgets(str, maxsize, _fin) == NULL) break; if (str[0] != '\n' && str[0] != '\r') break; }
	int slen = strlen(str); if (slen == 0) return 0;
	if (str[slen - 1] == '\n' || str[slen - 1] == '\r') str[--slen] = 0;
	return slen;
}

#define writec(var) fprintf(_fout,"%c",var)
#define writecsp(var) fprintf(_fout,"%c ",var)
#define writecln(var) fprintf(_fout,"%c\n",var)
#define writei(var) fprintf(_fout,"%d",var)
#define writeisp(var) fprintf(_fout,"%d ",var)
#define writellsp(var) fprintf(_fout,"%lld ",var)
#define writeiln(var) fprintf(_fout,"%d\n",var)
#define writellln(var) fprintf(_fout,"%lld\n",var)
#define writeulln(var) fprintf(_fout,"%llu\n",var)
#define writefln(var) fprintf(_fout,"%f\n",var)
#define writes(str) fprintf(_fout,"%s",str)
#define writesp() fprintf(_fout," ")
#define writeln() fprintf(_fout,"\n")
#define RUN_LOCAL(testfilename) {_fin = fopen(testfilename, "r"); if(_fin==NULL) _fin=stdin;}
#define swap(type,a, b) {type t=a;a=b;b=t;}
#define sort(data,n) std::sort(data,data+n)
//#define mod(a,b) b==0 ? 0 : a==0 ? 0 :(a>0 ? a%b : b + a % b) //what the hell is this...
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a, ll b) { ll g = gcd(a, b); return (a / g) * b; }
void reverse(char* data, int n) { int k = n >> 1; repi(k) { char tmp = data[i]; data[i] = data[n - i - 1]; data[n - i - 1] = tmp; } }
void reverse(int* data, int n) { int k = n >> 1; repi(k) { int tmp = data[i]; data[i] = data[n - i - 1]; data[n - i - 1] = tmp; } }

#define _Vec(type, name) \
struct name { type* data; int size; int n; };\
void init(name* t, int size) { t->data = (type*)malloc(sizeof(type) * size); t->size = size; t->n = 0; }\
void resize(name* t) { int ns = t->size * 1.2f; t->data = (type*)realloc(t->data, sizeof(type) * ns); t->size = ns; }\
void add(name* t, type val) { if (t->n >= t->size) resize(t); int k = t->n; t->data[k] = val; t->n = k + 1; }\
void free(name* t) { free(t->data);}
_Vec(int, Veci)
_Vec(long long, Vecll)
_Vec(char*, Vecs)

#define _ispal(type, name) int name(type* a, type* b, int n){repi(n){if (a[i] != b[n - i - 1]){return 0;}}return 1;}
_ispal(int, ispali)
_ispal(char, ispalc)

#define _Pair(type, name) struct name{type x, y;}; \
int cmp_##name(const void* _a, const void* _b){name* a = (name*)_a;name* b = (name*)_b;if (a->x == b->x){if (a->y < b->y) return -1;else return 1;}if (a->x < b->x) return -1;else return 1;}\
void sort_##name(name* d, int n){qsort(d, n, sizeof(name), cmp_##name);}
	_Pair(int, Pairi)

	int cmp_Str(const void* _a, const void* _b) { char* a = *((char**)_a); char* b = *((char**)_b); return strcmp(a, b); }
void sort_Str(char** str, int n) { qsort(str, n, sizeof(char*), cmp_Str); }

ll expmod(ll x, ll n, ll m) {
	ll ans = 1;
	for (; n;) {
		if (n & 1) ans = (ans * x) % m;
		x = (x * x) % m;
		n >>= 1;
	}
	return ans;
}

ll combmod(ll n, ll k, ll m) {
	ll ret = 1;
	ll div = 1;
	for (ll i = 0; i < k; ++i) {
		ret = (ret * (n - i) % m) % m;
		div = (div * (i + 1)) % m;
	}
	div = expmod(div, m - 2, m) % m;
	return (ret * div) % m;
}

int next_perm(int* data, int n)
{
	//find first i,i+1 where d[i]<d[i+1]
	int rootidx = -1;
	for (int i = n - 2; i >= 0; --i)
	{
		if (data[i] < data[i + 1])
		{
			rootidx = i;
			break;
		}
	}

	if (rootidx == -1) return 0;

	int ceilingidx = rootidx + 1;
	//find smallest d[x] after d[rooti] where d[x]>d[rooti]
	for (int i = rootidx + 2; i < n; ++i)
	{
		if (data[i] > data[rootidx] && data[i] < data[ceilingidx])
		{
			ceilingidx = i;
		}
	}

	swap(int, data[rootidx], data[ceilingidx]);
	sort(&data[rootidx + 1], n - rootidx - 1);
	return 1;
}
//------------------------------------------

//------------
int main()
{

	RUN_LOCAL("xxxtestcase_06.txt");
	int n = readi();
	int a = readi();
	repi(n) readi();
	int ans = (n - a) / (a - 1);
	if ((n - a) % (a - 1) != 0)ans++;
	writeiln(ans+1);
	return 0;
}