#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <cstring>
#include <queue>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <map>
#include <numeric>
#include <functional>
#include <cmath>
#include <cassert>
#include <string>
#include <iostream>

using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef pair<ll, ll> PL;
const ll MOD = 1000000007;
const int mod = 1000000007;
ll INF = 1LL << 60;

template <typename T>
istream &operator>>(istream &is, vector<T> &vec)
{
   for (auto &v : vec)
      is >> v;
   return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec)
{
   os << "[";
   for (auto v : vec)
      os << v << ",";
   os << "]";
   return os;
}
template <typename T>
ostream &operator<<(ostream &os, const deque<T> &vec)
{
   os << "deq[";
   for (auto v : vec)
      os << v << ",";
   os << "]";
   return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &vec)
{
   os << "{";
   for (auto v : vec)
      os << v << ",";
   os << "}";
   return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &vec)
{
   os << "{";
   for (auto v : vec)
      os << v << ",";
   os << "}";
   return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &vec)
{
   os << "{";
   for (auto v : vec)
      os << v << ",";
   os << "}";
   return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &vec)
{
   os << "{";
   for (auto v : vec)
      os << v << ",";
   os << "}";
   return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &pa)
{
   os << "(" << pa.first << "," << pa.second << ")";
   return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const map<TK, TV> &mp)
{
   os << "{";
   for (auto v : mp)
      os << v.first << "=>" << v.second << ",";
   os << "}";
   return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp)
{
   os << "{";
   for (auto v : mp)
      os << v.first << "=>" << v.second << ",";
   os << "}";
   return os;
};
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val)
{
   fill((T *)array, (T *)(array + N), val);
}

struct BIT
{
    int n;
    vector<int> bit;
    BIT()
    {
        init();
    }
    BIT(int n) : n(n)
    {
        init();
    }
    void init()
    {
        bit.clear();
        bit.resize(n + 1, 0);
    }
    int sum(int i)
    {
        int s = 0;
        while (i > 0)
        {
            s += bit[i];
            i -= i & -i;
        }
        return s;
    }
    int sum(int x, int y)
    {
        return sum(y) - sum(x - 1);
    }
    void add(int i, int x)
    {
        while (i <= n)
        {
            bit[i] += x;
            i += i & -i;
        }
    }
 int lower_bound(int w)
    {
        if (w <= 0)
            return 0;
        int x = 0, r = 1;
        while (r < n)
            r <<= 1;
        for (int k = r; k > 0; k >>= 1)
        {
            if (x + k <= n && bit[x + k] < w)
            {
                w -= bit[x + k];
                x += k;
            }
        }
        return x + 1;
    }
};

//int a[202020], b[202020];
int solve()
{

   ll n,k;
   cin >> n >> k;
   vector<ll> a(n);
   cin >> a;
   auto it = find(a.begin(),a.end(),1);
   double d = (double)distance(a.begin(),it) + 1;
   cout << ceil((double)(n - 1) / (k - 1)) << endl;

   return 0;
}
int main()
{
   //cout.precision(10)
   ios::sync_with_stdio(false);
   cin.tie(0);

   solve();
}

Measured memory_bytes: 1024