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 <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream> 
#include <array>
#define _USE_MATH_DEFINES
#include <math.h>
#include <unordered_set>
#include<unordered_map>

using namespace std;


inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }


typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long long int> vll;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef unsigned long long ull;

//repetition
//------------------------------------------
#define REP(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  REP(i,0,n)
#define rrep(i,n) for(int i=(n);i>=0;i--)
#define VEC_2D(a,b) vector<vector<int> >(a, vector<int>(b, 0))
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define INF (1001000000)
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define UNIQ(c) (c).erase(unique((c).begin(),(c).end()), (c).end());
#define MOD 1000000007LL
#define MS(v,n) memset((v),(n),sizeof(v))


//input
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;

//output
#define P(p) cout<<(p)<<endl;
#define FSP(a) cout << fixed << setprecision(a)

template<typename T> T gcd(T x, T y) {
	if (y == 0) return x;
	else return gcd(y, x%y);
}

template<typename T> T lcm(T a, T b) {
	return a / gcd(a, b) * b;
}

template<typename T> bool is_prime(T n) {
	for (int i = 2; i * i <= n; i++) {
		if (n % i == 0) return false;
	}
	return n != 1;
}

map<int, int> prime_factor(int n) {
	map<int, int> res;
	for (int i = 2; i * i <= n; i++) {
		while (n % i == 0) {
			++res[i];
			n /= i;
		}
	}
	if (n != 1) res[n] = 1;
	return res;
}

int extgcd(int a, int b, int& x, int& y) {//
	int d = a;
	if (b != 0) {
		d = extgcd(b, a%b, y, x);
		y -= (a / b)*x;
	}
	else {
		x = 1; y = 0;
	}
	return d;
}



ll mod_pow(ll x, ll n, ll mod) {
	if (n == 0) return 1;
	ll res = mod_pow(x * x % mod, n / 2, mod);
	if (n & 1) res = res * x % mod;
	return res;
}
vector<string> split(const string &str, char delim) {
	vector<string> res;
	size_t current = 0, found;
	while ((found = str.find_first_of(delim, current)) != string::npos) {
		res.push_back(string(str, current, found - current));
		current = found + 1;
	}
	res.push_back(string(str, current, str.size() - current));
	return res;
}

bool is_kadomatsu(int a, int b, int c) {
	if (a == b || a == c || b == c)return false;
	if (a > b && c > b) return true;
	if (a < b && c < b)return true;
	return false;
}


struct UF {
	int n;
	vi d;
	UF() {}
	UF(int n) :n(n), d(n, -1) {}
	int root(int v) {
		if (d[v] < 0) return v;
		return d[v] = root(d[v]);
	}
	bool same(int a, int b) {
		return root(a) == root(b);
	}
	bool unite(int x, int y) {
		x = root(x); y = root(y);
		if (x == y) return false;
		if (size(x) < size(y)) swap(x, y);
		d[x] += d[y];
		d[y] = x;
		return true;
	}
	int size(int v) { return -d[root(v)]; }
};



vector<int> divisor(int n) {
	if (n == 1) return{};
	vi res;
	for (int i = 1; i*i <= n; i++) {
		if (n%i == 0) {
			res.emplace_back(i);
			if (i != 1 && i != n / i)res.emplace_back(n / i);
		}
	}
	return res;
}

struct Bellmanford {
	int n;
	struct edge {
		int from, to, cost;
	};
	vector<edge> E;
	vi d;
	Bellmanford(int n) :n(n), d(n) {
		E.resize(n);
	}
	void add_edge(int x, int y, int cost) {
		edge e;
		e.from = x; e.to = y; e.cost = cost;
		E.push_back(e);
	}
	void shortest_path(int s) {
		rep(i, n)d[i] = INF;
		d[s] = 0;
		while (true) {
			bool update = false;
			for (auto e : E) {
				if (d[e.from] != INF && d[e.to] > d[e.from] + e.cost) {
					d[e.to] = d[e.from] + e.cost;
					update = true;
				}
			}
			if (!update) break;
		}
	}
};
struct Dijkstra {
	int n;
	struct edge {
		int to; ll cost;
	};
	vector<vector<edge>> G;
	vll d;
	Dijkstra(int n) :n(n), d(n) {
		G.resize(n);
	}
	void add_edge(int x, int y, ll cost) {
		edge e;
		e.to = y; e.cost = cost;
		G[x].push_back(e);
	}
	void shortest_path(int s) {
		rep(i, n)d[i] = 100000000000000000;
		d[s] = 0;
		priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<pair<ll, int>>> que;
		que.push(make_pair(0, s));

		while (!que.empty()) {
			pii p = que.top(); que.pop();
			int v = p.second;
			if (d[v] < p.first) continue;
			for (auto e : G[v]) {
				if (d[e.to] > d[v] + e.cost) {
					d[e.to] = d[v] + e.cost;
					que.push(make_pair(d[e.to], e.to));
				}
			}
		}
	}
};

struct Segmenttree {
	int n;
	vector<int> dat;
	Segmenttree(int n_) {
		n = 1;
		while (n < n_) n *= 2;
		dat = vector<int>(2 * n - 1, 0);
	}

	void add(int idx, ll val) {//0-indexed
		idx += n - 1;
		dat[idx]+=val;
		while (idx > 0) {
			idx = (idx - 1) / 2;
			dat[idx]+=val;
		}
	}
	int query(int a, int b) {
		return query_seg(a, b, 0, 0, n);
	}
	int query_seg(int a, int b, int k, int l, int r) {
		if (r <= a || b <= l) return 0;
		if (a <= l && r <= b)return dat[k];
		else {
			return query_seg(a, b, k * 2 + 1, l, (l + r) / 2) + query_seg(a, b, k * 2 + 2, (l + r) / 2, r);
		}
	}
};

struct Trie {
	Trie *next[26];

	Trie() {
		fill(next, next + 26, (Trie *)0);
	}

	void insert(char *s) {
		if (*s == '\0') return;
		if (this->next[*s - 'a'] == NULL) {
			this->next[*s - 'a'] = new Trie();
		}
		this->next[*s - 'a']->insert(s + 1);
	}

	bool find(char *s) {
		if (*s == '\0') return true;
		if (this->next[*s - 'a'] == NULL) {
			return false;
		}
		return this->next[*s - 'a']->find(s + 1);
	}
};


	struct edge { int to, cap, rev; };

	vector<edge> G[200005];
	int level[200005];
	int iter[200005];

	void add_edge(int from, int to, int cap) {
		G[from].push_back({ to, cap, (int)G[to].size() });
		G[to].push_back({ from, 0, (int)G[from].size() - 1 });
	}
	void fbfs(int s) {
		memset(level, -1, sizeof(level));
		queue<int> que;
		level[s] = 0;
		que.push(s);
		while (!que.empty()) {
			int v = que.front(); que.pop();
			for(edge &e : G[v]){
				if (e.cap > 0 && level[e.to] < 0) {
					level[e.to] = level[v] + 1;
					que.push(e.to);
				}
			}
		}
	}
	int fdfs(int v, int t, int f) {
		if (v == t) return f;
		for (edge &e : G[v]) {
			if (e.cap > 0 && level[v] < level[e.to]) {
				int d = fdfs(e.to, t, min(f, e.cap));
				if (d > 0) {
					e.cap -= d;
					G[e.to][e.rev].cap += d;
					return d;
				}
			}
		}
		return 0;
	}
	int max_flow(int s, int t) {
		int flow = 0;
		for (;;) {
			fbfs(s);
			if (level[t] < 0) return flow;
			memset(iter, 0, sizeof(iter));
			int f;
			while ((f = fdfs(s, t, INF)) > 0) {
				flow += f;
			}
		}
	}


//------------------------


int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	int n, k;
	cin >> n >> k;
	vi a(n);
	rep(i, n)cin >> a[i];

	if (k == n) {
		P(1);
		return 0;
	}
	int mn = 1111111;
	int id = 0;
	rep(i, n) {
		if (mn < a[i]) {
			mn = a[i];
			id = i;
		}
	}
	int ans = (id+k-2) / (k - 1);
	ans += (n - 1 - id + k - 2) / (k - 1);
	P(ans);
	return 0;
}