#include <iostream> // cin, cout, string
#include <algorithm> // sort, min/max, upper/lower_bound, transform, count/copy_if
#include <vector>
#include <iomanip> // setprecision
#include <locale>
#include <cstring>

using namespace std;

#define int long long

using i64 = int64_t;

using pii = pair<int, int>;
using ppi = pair<pii, int>;
using pip = pair<int, pii>;
using ppp = pair<pii, pii>;

constexpr int IINF =  0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
// constexpr int INF = sizeof(int) == sizeof(long long) ? LINF : IINF;
constexpr long long MOD = 1e9+7;


#define _overload(_1,_2,_3,name,...) name
#define _rep1(i,a,b) for(int i=(a), _##i=(b);i<_##i;++i)
#define _rep2(i,n) _rep1(i,0,n)
#define _rep3(n) _rep2(i,n)
#define rep(...) _overload(__VA_ARGS__,_rep1,_rep2,_rep3)(__VA_ARGS__)
#define _rrep1(i,a,b) for(int i=(b)-1, _##i=(a);i>=_##i;--i)
#define _rrep2(i,n) _rrep1(i,0,n)
#define _rrep3(n) _rrep2(i,n)
#define rrep(...) _overload(__VA_ARGS__,_rrep1,_rrep2,_rrep3)(__VA_ARGS__)

#define each(i,a) for(auto&& i:a)
#define each2(i,a) for(const auto& i:a)

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sz(a) (int)(a).size()
#define rev(a) reverse(all(a))
#define rs(a,l) (a).resize(l)

#define upper(s) transform(all(s), (s).begin(), toupper)
#define lower(s) transform(all(s), (s).begin(), tolower)

#define sum(a) accumulate(all(a), 0)
#define unless(a) if(!(a))
#define qt(a) #a

#define endl '\n'
#define dec(n) fixed << setprecision(n)
#define zfl(n)  setfill(‘0’) << setw(n)

#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

#define cauto const auto

template<typename T> inline constexpr T gcd(T a, T b) { if(a==0 || b==0) return 0; while(b) { a = a%b; swap(a, b); } return a; }
template<typename T> inline constexpr T extgcd2(T a, T b, T &x, T &y) { x = 1; y = 0; while (b) { T t = a / b; a -= t * b; swap(a, b); x -= t * y; swap(x, y); } return a; }
template<typename T> inline constexpr T lcm(T a, T b) { return a / gcd(a, b) * b; }

template<typename T> inline constexpr bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<typename T> inline constexpr bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

template<typename A, typename B> istream &operator>>(istream &is, pair<A, B> &p) { is >> p.first; is >> p.second; return is; }
template<typename A, typename B> ostream &operator<<(ostream &os, const pair<A, B> &p) { os << p.first << " " << p.second; return os; }

template<typename T> istream &operator>>(istream &is, vector<T> &v) { for(auto&& x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &v) { for(int i=0; i<(int)v.size(); ++i) cout << v[i] << ( i+1 != (int)v.size() ? " " : "" ); return os; }

#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg> void __f(const char* name, const Arg& arg) { cerr << name << " : " << arg << endl; }
template <typename Arg, typename... Args> void __f(const char* names, const Arg& arg, const Args&... args)
{ const char* comma = strchr(names + 1, ','); cerr.write(names, comma - names) << " : " << arg << " | "; __f(comma+1, args...); }

inline auto yes(const bool &c) { return c ? "yes" : "no"; }
inline auto Yes(const bool &c) { return c ? "Yes" : "No"; }
inline auto YES(const bool &c) { return c ? "YES" : "NO"; }
inline auto Possible(const bool &c) { return c ? "Possible" : "Impossible"; }
inline auto POSSIBLE(const bool &c) { return c ? "POSSIBLE" : "IMPOSSIBLE"; }

constexpr int lim = 100007; // 10^5+7

int N,K;
vector<int> A;
signed main() {
  cin.tie(0); cout.tie(0);
  ios_base::sync_with_stdio(false);
  cin >> N >> K;
  A.resize(N);
  cin >> A;

  int idx1 = 0;
  rep(i,A.size()) {
    if (A[i] == 1) {
      idx1 = i;
      break;
    }
  }

  int ans = LINF;
  rep (i, K) {
    int start = idx1 + i;
    if (start >= N) break;
    int sum = 0;
    int K1 = K - 1;
    if (start > 0) {
      sum += (start - 1 + 1 + K1 - 1) / K1;
    }
    if (start < N - 1) {
      sum += (N - (start + 1) + K1 - 1 ) / K1;
    }
    chmin(ans, sum);
  }
  cout << ans << endl;

  return 0;
}

Measured memory_bytes: 1024