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

using ll = long long;

#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define pb push_back
#define mp make_pair
#define fill(x, y) memset(x, y, sizeof(x))
#define even(x) x % 2 == 0
#define odd(x) x % 2 != 0
#define all(x) x.begin(), x.end()
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define F first
#define S second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

// these functions return the position of result of Binary Search.
#define LB(s, t, x) (int) (lower_bound(s, t, x) - s)
#define UB(s, t, x) (int) (upper_bound(s, t, x) - s)

ll qp(ll a, ll b, int mo) { ll ans = 1; do { if (b & 1) ans = 1ll * ans * a % mo; a = 1ll * a * a % mo; } while (b >>= 1); return ans; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
int dx8[] = { 1, -1, 0, 0, 1, 1, -1, -1 }, dy8[] = { 0, 0, -1, 1, -1, 1, -1, 1 };

template <typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts>
auto make_v(size_t a, size_t b, Ts... ts) { return vector<decltype(make_v<T>(b, ts...))>(a, make_v<T>(b, ts...)); }
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T &t, const V &v) { t = v; }
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T &t, const V &v) { for (auto &e : t) fill_v(e, v); }
// auto dp = make_v<int>(4, h, w);
// fill_v(dp, 0);

const ll INF_LL = (1ll << 60);
const int INF_INT = (int)1e9;
const ll MOD_CONST = (ll)(1e9 + 7);

template <class T>
bool chmax(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template <class T>
bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }

inline tuple<ll, ll> rotate45(tuple<ll, ll> point) { ll x = get<0>(point), y = get<1>(point); return tuple<ll, ll>(x + y, x - y); }
inline bool rangeCheck2D(int nx, int ny, int Width, int Height) { return nx >= 0 and nx < Width and ny >= 0 and ny < Height; }
void out_sequence(vector<ll> &out) { REP(i, out.size()) { cout << out[i]; if (i == out.size() - 1) cout << endl; else cout << " "; } }

// 1≤k≤n≤10^7
class CombLib {
private:
	int mMax, mMod;
	vector<ll> fac, finv, inv;
public:
	CombLib(int modNum = 1000000007, int maxSize = 510000) {
		mMax = maxSize, mMod = modNum;
		fac = vector<ll>(maxSize);
		finv = vector<ll>(maxSize);
		inv = vector<ll>(maxSize);
	    fac[0] = fac[1] = 1;
    	finv[0] = finv[1] = 1;
    	inv[1] = 1;
    	for (int i = 2; i < mMax; i++){
	        fac[i] = fac[i - 1] * i % mMod;
    	    inv[i] = mMod - inv[mMod%i] * (mMod / i) % mMod;
        	finv[i] = finv[i - 1] * inv[i] % mMod;
    	}
	}

	ll calc(int n, int k){
    	if (n < k) return 0;
    	if (n < 0 || k < 0) return 0;
    	return fac[n] * (finv[k] * finv[n - k] % mMod) % mMod;
	}
};

int main(void)
{
	cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(20);

	ll N, K; cin >> N >> K;
	REP(i, N) {
      int t; cin >> t;
    }
	cout << int(ceil(double(N-1) / double(K-1))) << endl;



	return 0;
}


Measured memory_bytes: 256