#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<algorithm>
#include<numeric>
#include<limits>
#include<bitset>
#include<functional>
#include<type_traits>
#include<queue>
#include<stack>
#include<array>
#include<random>
#include<utility>

#include<boost/multi_array.hpp>
#include<boost/variant.hpp>
#include<boost/optional.hpp>
#include<boost/optional/optional_io.hpp>
#include<boost/range/irange.hpp>
#include<boost/range/algorithm.hpp>
#include<boost/range/adaptors.hpp>
#include<boost/range/size.hpp>
#include<boost/multiprecision/cpp_int.hpp>

#include<cstdlib>
#include<ctime>

#define DEFINE_STRUCT2(arg0, arg1, stc)\
auto const& arg0=std::get<0>(stc);\
auto const& arg1=std::get<1>(stc)

#define DEFINE_STRUCT3(arg0, arg1, arg2, stc)\
DEFINE_STRUCT2(arg0, arg1, stc);\
auto const& arg2=std::get<2>(stc)

namespace lib
{
	using namespace boost::range;
	template<class T>constexpr T pow(T base, std::size_t p)
	{
		if (p == 0)
		{
			return T(1);
		}
		else if (p == 1)
		{
			return base;
		}
		else if (p == 2)
		{
			return base * base;
		}
		else if (p % 2 == 0)
		{
			return pow(pow(base, p / 2), 2);
		}
		else
		{
			return pow(pow(base, p / 2), 2)*base;
		}
	}

	template<class T>constexpr T gcd(T a, T b)
	{
		return a > b ? gcd(b, a) : a == T() ? b : gcd(b%a, a);
	}

	constexpr int intlog2(std::int64_t x)
	{
		std::int64_t j = 2;
		for (auto i = 0; i < 63; ++i)
		{
			if (j > x)
			{
				return int(i);
			}
			j <<= 1;
		}
		return 63;
	}

	template<class T>using p_queue = std::priority_queue<T, std::vector<T>, std::greater<>>;

	struct union_find_light
	{
		std::vector<int> upper;
		union_find_light(std::size_t size):upper(size, -1) {}

		int group(int index)
		{
			if (upper[index] == -1)
			{
				return index;
			}
			else
			{
				auto ret = group(upper[index]);
				upper[index] = ret;
				return ret;
			}
		}

		bool merge(int x, int y)
		{
			auto gx = group(x);
			auto gy = group(y);
			if (gx != gy)
			{
				upper[gx] = gy;
				return true;
			}
			return false;
		}

		auto get()
		{
			std::map<int, std::set<int>> ret;
			for (int i = 0; i < upper.size(); ++i)
			{
				ret[group(i)].emplace(i);
			}
			return ret;
		}

		auto tops()
		{
			std::set<int> ret;
			for (int i = 0; i < upper.size(); ++i)
			{
				ret.emplace(group(i));
			}
			return ret;
		}

		bool is_same_group(int x, int y)
		{
			return group(x) == group(y);
		}
	};

	template<class Selector>auto make_compare(Selector selector)
	{
		return [=](auto const& lhs, auto const& rhs) {return selector(lhs) < selector(rhs); };
	}

	template<class Iterator>auto make_range(Iterator first, Iterator last)
	{
		struct return_type
		{
			Iterator first;
			Iterator last;
			auto begin()const
			{
				return first;
			}
			auto end()const
			{
				return last;
			}
		};
		return return_type{ first, last };
	}

	template<class T>auto factorial_array(std::size_t max)
	{
		std::vector<T> ret(max + 1);
		ret[0] = 1;
		for (auto i : boost::irange<std::size_t>(1, max + 1))
		{
			ret[i] = ret[i - 1] * T(i);
		}
		return ret;
	}

	auto const first = [](auto p) {return p.first; };
	auto const second = [](auto p) {return p.second; };

	auto maxf = [](auto a, auto b) {return std::max(a, b); };
	auto minf = [](auto a, auto b) {return std::min(a, b); };

	template<class T>using dual_array = boost::multi_array<T, 2>;
	template<class T>using optional_dual_array = boost::multi_array<boost::optional<T>, 2>;
	namespace detail
	{
		template<class T>struct graph_t
		{
			typedef std::vector<std::map<int, T>> type;
		};
		template<>struct graph_t<void>
		{
			typedef std::vector<std::set<int>> type;
		};

	}
	template<class T = void>using graph = typename detail::graph_t<T>::type;
	template<class T>constexpr T min_value = std::numeric_limits<T>::min();
	template<class T>constexpr T max_value = std::numeric_limits<T>::max();
	
}

namespace out
{
	namespace detail
	{
		template<class>struct void_out
		{
			typedef void type;
		};
		template<class T>auto write(std::ostream& ost, T const& val)
			->typename void_out<decltype(ost << val)>::type
		{
			ost << val;
		}
		template<class T, class... Ts>void write(std::ostream& ost, T const& val, Ts const&... nexts)
		{
			write(ost, val);
			ost << " ";
			write(ost, nexts...);
		}
		template<class T, std::size_t... Is>void write_tuple(std::ostream& ost, T const& tuple, std::index_sequence<Is...>)
		{
			write(ost, std::get<Is>(tuple)...);
		}
		template<class... Ts>void write(std::ostream& ost, std::tuple<Ts...>const& tuple)
		{
			write_tuple(ost, tuple, std::make_index_sequence<sizeof...(Ts)>());
		}
		template<class T0, class T1>void write(std::ostream& ost, std::pair<T0, T1>const& pair)
		{
			write(ost, pair.first, pair.second);
		}
		template<class T>void write(std::ostream& ost, boost::optional<T>const& opt)
		{
			if (opt)
			{
				write(ost, *opt);
			}
			else
			{
				write(ost, "--");
			}
		}
		template<class T>void write(std::ostream& ost, std::vector<T>const& vec)
		{
			auto size = vec.size();
			if (size == 0)
			{
				return;
			}
			write(ost, vec[0]);
			for (auto i : boost::irange<std::size_t>(1, size))
			{
				ost << " ";
				write(ost, vec[i]);
			}
		}
		template<class T>void write(std::ostream& ost,lib::dual_array<T> const& ar)
		{
			auto a = ar.shape()[0];
			auto b = ar.shape()[1];
			write(ost, ar[0][0]);
			for (auto i : boost::irange<std::size_t>(1, b))
			{
				write(ost, " ", ar[0][i]);
			}
			for (auto i : boost::irange<std::size_t>(1, a))
			{
				ost << std::endl;
				write(ost, ar[i][0]);
				for (auto j : boost::irange<std::size_t>(1, b))
				{
					write(ost, " ", ar[i][j]);
				}
			}
		}
	}
	void write_line()
	{
		std::cout << std::endl;
	}
	template<class...Ts>void write_line(Ts const&... args)
	{
		detail::write(std::cout, args...);
		std::cout << std::endl;
	}
	void debug()
	{
		std::cerr << std::endl;
	}
	template<class...Ts>void debug(Ts const&... args)
	{
		detail::write(std::cerr, args...);
		std::cerr << std::endl;
	}
	template<class Range>void write_range(Range const& rng)
	{
		for (auto const& v : rng)
		{
			write_line(v);
		}
	}
	template<class Range>void debug_range(Range const& rng)
	{
		for (auto const& v : rng)
		{
			debug(v);
		}
	}
}

namespace adaptor = boost::adaptors;

typedef std::pair<std::int64_t, std::int64_t> int64pair;

void Main()
{
	int N, K;
	std::cin >> N >> K;
	int t;
	for (int i : boost::irange(0, N))
	{
		int a;
		std::cin >> a;
		if (a == 1)
		{
			t = i;
		}
	}
	int min = t;
	int max = t;
	int count = 0;
	while (min > 0)
	{
		if (min > K - 1)
		{
			min -= K - 1;
			++count;
		}
		else
		{
			max += K - 1 - min;
			min = 0;
			++count;
		}
	}
	while (max < N - 1)
	{
		max += K - 1;
		++count;
	}
	out::write_line(count);
}

int main()
{
	std::cin.tie(nullptr);
	std::cin.sync_with_stdio(false);
	std::cout << std::fixed;
	Main();
}

Measured memory_bytes: 256