//Iamskk//
#include <iostream>
#include <iosfwd>
#include <iomanip>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <cassert>
#include <cctype>
#include <climits>
#include <vector>
#include <bitset>
#include <set>
#include <queue>
#include <stack>
#include <map>
#include <deque>
#include <string>
#include <list>
#include <iterator>
#include <sstream>
#include <complex>
#include <fstream>
#include <functional>
#include <numeric>
#include <utility>
#include <algorithm>
#include <assert.h>
#include <unordered_map>
using namespace std;
#define IOS ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(0);
#define endl '\n'
#define yes cout<<"YES"<<endl
#define no cout<<"NO"<<endl
#define pb push_back
#define all(c) (c).begin(),(c).end()
#define rall(c) (c).rbegin(),(c).rend()
#define ai(arr,n) for(int i=0;i<n;i++)cin>>arr[i];
#define ao(arr) for(auto asdfasdfasdf:arr) cout<<asdfasdfasdf<<" ";
#define mi(arr,m,n) for(int i=0;i<m;i++){ for(int j=0;j<n;j++) cin>>arr[i][j];}
#define mo(arr,m,n) for(int i=0;i<m;i++){ for(int j=0;j<n;j++) cout<<arr[i][j]<<" "; cout<<endl;}
#define countsetbits(x) __builtin_popcount(x)
#define ll long long
#define debug cout<<"I AM EXECUTING"<<endl
#define testcases int asdf; cin>>asdf; while(asdf--)
#define vi vector<int>
#define si set<int>
#define vc vector<char>
#define sc set<char>
#define vll vector<long long int>
#define vs vector<string>
#define vpp vector<pair<int,int>> 
#define vppo(prs) for(auto x:prs){cout<<x.first<<" "<<x.second<<endl;}
#define For(__,$$,adfdf) for(int __ = $$; __<adfdf;__++)
//In the range = p[r]-p[l+1]; p.size()==n+1.
vll  primeFactors(ll int n)  {  vll a;
    while (n % 2 == 0)  {  a.pb(2);
        n = n/2;  
    }for (int i = 3; i <= sqrt(n); i = i + 2)  {  
        while (n % i == 0)  {  a.pb(i);n = n/i;  
		}
	}    if (n > 2)  a.pb(n);
    return a;
}  
vll printDivisors(ll int n) 
{ vll v;for (ll int i = 1; i <= sqrt(n); i++) { if (n % i == 0) { 
if (n / i == i)  
v.pb(i); 
else { 
v.pb(i);
v.push_back(n / i); 
} 
} 
} 
      return v; 
} 
ll int mpow(ll int base,ll  int exp,ll int mod) {
	base %= mod;
	ll int result = 1ll;
	while (exp > 0) {
	if (exp & 1ll) result = ((ll)result * base) % mod;
	base = ((ll)base * base) % mod;
	exp >>= 1;
	}
	return result;
}
string sconvert(ll int n)
{
	stringstream ss;
	ss<<n;
	string str = ss.str();
	return str;
}
bool sortbysec(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.second > b.second); 
}
template<typename KeyType, typename ValueType>  // for finding max element in the map.
std::pair<KeyType,ValueType> get_max( const std::map<KeyType,ValueType>& x ) {
  using pairtype=std::pair<KeyType,ValueType>; 
  return *std::max_element(x.begin(), x.end(), [] (const pairtype & p1, const pairtype & p2) {
        return p1.second < p2.second;
  }); 
}
const ll int mod = 1e9+7;
ll modmul(ll a, ll b) {
    return ((a%mod) * (b%mod)) % mod;
}
 
ll modadd(ll a , ll b){
    return((a%mod)+(b%mod)+mod)%mod;
}
 
ll modsub(ll a , ll b){
    return((a%mod) - (b%mod) + mod)%mod;
}

 
void  single()
{
     int n;
     cin>>n;
     int k;
     cin>>k;
     vi arr(n);
     ai(arr,n);
	 if(k==n){
		 cout<<1<<endl;
		 return;
	 }
	 else{
		 
		 cout<<ceil((double)(n-k)/(k-1))+1<<endl;
	 }
	 return;
}
void multiple()
{
	testcases
    {
		single();
	}
}

int main()
{
  IOS;
 // freopen("input.txt","r",stdin);
  //freopen("output.txt","w",stdout);
  single();
}















Measured memory_bytes: 3780