/*
* じょえチャンネル
* 高評価・チャンネル登録よろしくおねがいします！
* https://www.youtube.com/channel/UCRXsI3FL_kvaVL9zoolBfbQ
*/

#include <bits/stdc++.h>

#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#define f(i,n) for(int i=0;i<(n);i++)


#define inf (int)(3e18)

//here!!!
// define int long long !!!!!

#define int long long

// define int long long !!!!!


#define mod (int)((1e9)+7)

#define intt long long
#define itn long long
#define P pair<int,int>

#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,n) for(int i=1;i<=n;i++)

#define ALL(v) v.begin(),v.end()

using namespace std;

//Library
//モッドパウ
inline int modpow(int x, int y, int m = mod) {
    int res = 1;
    while (y) {
        if (y % 2) {
            res *= x;
            res %= m;
        }
        x = x * x % m;
        y /= 2;
    }
    return res;
}

int mypow(int x, int y) {
    int res = 1;
    while (y) {
        if (y % 2) {
            res *= x;
        }
        x = x * x;
        y /= 2;
    }
    return res;
}
//is the number (x) a prime number?
bool prime(int x) {
    for (int i = 2; i * i <= x; i++) {
        if (!(x % i)) {
            return false;
        }
    }
    return true;
}


//saidai-kouyakusuu
inline int gcd(int x, int y) {
    if (!y) {
        return x;
    }
    return gcd(y, x % y);
}


//number of keta
int keta(int x) {
    int ans = 0;
    while (x) {
        x /= 10;
        ans++;
    }
    return ans;
}

//sum of keta
int ketasum(int x) {
    int ans = 0;
    while (x) {
        ans += x % 10;
        x /= 10;
    }
    return ans;
}

inline int lcm(int x, int y) {
    int ans = x / gcd(x, y) * y;
    return ans;
}
int twobeki(int x) {
    int ans = 0;
    while (1) {
        if (!(x & 1)) {
            ans++;
            x /= 2;
        }
        else {
            break;
        }
    }
    return ans;
}


template <class T, class U>
inline bool chmax(T& lhs, const U& rhs) {
    if (lhs < rhs) {
        lhs = rhs;
        return 1;
    }
    return 0;
}
template <class T, class U>
inline bool chmin(T& lhs, const U& rhs) {
    if (lhs > rhs) {
        lhs = rhs;
        return 1;
    }
    return 0;
}
void Yes(){
    cout<<"Yes"<<endl;
}
void No(){
    cout<<"No"<<endl;
}
void YES(){
    cout<<"YES"<<endl;
}
void NO(){
    cout<<"NO"<<endl;
}


#define fin(i) scanf("%lld",&i)
#define fout(i) printf("%lld",i)
#define fendl printf("\n")
//Library-End

class modInt {
    int value, modulo;
public:
    modInt() : value(0), modulo(mod) { value = 0; }
    template<typename T>
    modInt(T value = 0, int modulo = mod) : value(value), modulo(modulo) {
        if (value < 0) {
            value = -value;
            value %= modulo;
            value = -value;
            value += modulo;
        }
        this->value = value % modulo;
    }
    inline operator int()const { return value; }
    inline modInt& operator+=(modInt x) {
        value += x.value;
        if (value >= modulo)value -= modulo;
        return *this;
    }
    inline modInt& operator++() {
        if (value == modulo - 1)value = 0;
        else value++;
        return *this;
    }
    inline modInt& operator-()const {
        return modInt(0) -= *this;
    }
    inline modInt& operator-=(modInt x) {
        value -= x.value;
        if (value < 0)value += modulo;
        return *this;
    }
    inline modInt& operator--() {
        if (value == 0)value = modulo - 1;
        else value--;
        return *this;
    }
    inline modInt& operator*=(modInt x) {
        value = value * x.value % modulo;
        return *this;
    }
    inline modInt& operator/=(modInt x) {
        return operator*=(x.inv());
    }
    inline modInt inv() {
        return modpow(*this, modulo - 2);
    }
    template<typename T> modInt operator+(T x) { return modInt(*this) += x; }
    template<typename T> modInt& operator+=(T x) { return operator+=(modInt(x, modulo)); }
    template<typename T> modInt operator-(T x) { return modInt(*this) -= x; }
    template<typename T> modInt& operator-=(T x) { return operator-=(modInt(x, modulo)); }
    template<typename T> modInt operator*(T x) { return modInt(*this) *= x; }
    template<typename T> modInt& operator*=(T x) { return operator*=(modInt(x, modulo)); }
    template<typename T> modInt operator/(T x) { return modInt(*this) /= x; }
    template<typename T> modInt& operator/=(T x) { return operator/=(modInt(x, modulo)); }
};
istream& operator>>(istream& ist, modInt& x) {
    int a;
    ist >> a;
    x = a;
    return ist;
}
#define vecin(v) for(int i=0;i<v.size();i++)scanf("%lld",&v[i]);
#define vecout(v) {printf("%lld",v[0]);for(int i=1;i<v.size();i++)printf(" %lld",v[i]);printf("\n");}
//#define endl "\n" //interactive の時に注意！！！
//SegTree
template <class T>
class SegTree {
    int n;                       // 葉の数
    vector<T> data;              // データを格納するvector
    T def;                       // 初期値かつ単位元
    function<T(T, T)> operation; // 区間クエリで使う処理
    function<T(T, T)> update;    // 点更新で使う処理
    
    // 区間[a,b)の総和。ノードk=[l,r)に着目している。
    T _query(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) return def; // 交差しない
        if (a <= l && r <= b)
            return data[k]; // a,l,r,bの順で完全に含まれる
        else {
            T c1 = _query(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
            T c2 = _query(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
            return operation(c1, c2);
        }
    }
    
public:
    // _n:必要サイズ, _def:初期値かつ単位元, _operation:クエリ関数,
    // _update:更新関数
    SegTree(size_t _n, T _def, function<T(T, T)> _operation,
            function<T(T, T)> _update)
    : def(_def), operation(_operation), update(_update) {
        n = 1;
        while (n < _n) {
            n *= 2;
        }
        data = vector<T>(2 * n - 1, def);
    }
    
    // 場所i(0-indexed)の値をxで更新
    void change(int i, T x) {
        i += n - 1;
        data[i] = update(data[i], x);
        while (i > 0) {
            i = (i - 1) / 2;
            data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
        }
    }
    
    // [a, b)の区間クエリを実行
    T query(int a, int b) {
        return _query(a, b, 0, 0, n);
    }
    
    // 添字でアクセス
    T operator[](int i) {
        return data[i + n - 1];
    }
};

#define R_MIN ([](long long a, long long b) { return min(a, b); })
#define R_MAX ([](long long a, long long b) { return max(a, b); })
#define R_SUM ([](long long a, long long b) { return a + b; })

#define NORMAL_UPDATE ([](long long a, long long b) { return b; })
#define ADD_UPDATE ([](long long a, long long b) { return a + b; })
#define MINUS_UPDATE ([](long long a, long long b) { return a - b; }

class Union_Find {
    vector<int> par;
    vector<int> rankmy;
    vector<int> ookisa;
    
public:
    Union_Find(int size) {
        par = vector<int>(size);
        rankmy = vector<int>(size);
        ookisa=vector<int>(size);
        for (int i = 0; i < size; i++) {
            par[i] = i;
            ookisa[i]=1;
        }
    }
    
    int find(int x) {
        if (par[x] == x) {
            return x;
        }
        return par[x] = find(par[x]);
    }
    
    void unite(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y) {
            return;
        }
        if (rankmy[x] < rankmy[y]) {
            par[x] = y;
            ookisa[y]+=ookisa[x];
            ookisa[x]=0;
        }
        else {
            par[y] = x;
            ookisa[x]+=ookisa[y];
            ookisa[y]=0;
            if (rankmy[x] == rankmy[y]) {
                rankmy[x]++;
            }
        }
    }
    int size(int i){
        i=find(i);
        return ookisa[i];
    }
    bool same(int x, int y) {
        return find(x) == find(y);
    }
    
};

//Union-Find-End

int n,k,a[100004];
signed main(){
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cin>>n>>k;
    rep(i,n)cin>>a[i];
    cout<<(n+k-3)/(k-1)<<endl;
}






Measured memory_bytes: 1024