#include<bits/stdc++.h>
#include <cstring>
#include <iostream>
#define pie acos(-1)
#define si(a) scanf("%d",&a)
#define sii(a,b) scanf("%d %d",&a,&b)
#define siii(a,b,c) scanf("%d %d %d",&a,&b,&c)
#define sl(a) scanf("%lld",&a)
#define sll(a,b) scanf("%lld %lld",&a,&b)
#define slll(a,b,c) scanf("%lld %lld %lld",&a,&b,&c)
#define ss(st) scanf("%s",st)
#define sch(ch) scanf("%ch",&ch)
#define ps(a) printf("%s",a)
#define newLine() printf("\n")
#define pi(a) printf("%d",a)
#define pii(a,b) printf("%d %d",a,b)
#define piii(a,b,c) printf("%d %d %d",a,b,c)
#define pl(a) printf("%lld",a)
#define pll(a,b) printf("%lld %lld",a,b)
#define plll(a,b,c) printf("%lld %lld %lld",a,b,c)
#define pd(a) printf("%lf",a)
#define pdd(a,b) printf("%lf %lf",a,b)
#define pddd(a,b,c) printf("%lf %lf %lf",a,b,c)
#define pch(c) printf("%ch",c)
#define debug1(str,a) printf("%s=%d\n",str,a)
#define debug2(str1,str2,a,b) printf("%s=%d %s=%d\n",str1,a,str2,b)
#define debug3(str1,str2,str3,a,b,c) printf("%s=%d %s=%d %s=%d\n",str1,a,str2,b,str3,c)
#define debug4(str1,str2,str3,str4,a,b,c,d) printf("%s=%d %s=%d %s=%d %s=%d\n",str1,a,str2,b,str3,c,str4,d)
#define for0(i,n) for(i=0;i<n;i++)
#define for1(i,n) for(i=1;i<=n;i++)
#define forab(i,a,b) for(i=a;i<=b;i++)
#define forstl(i, s) for (__typeof ((s).end ()) i = (s).begin (); i != (s).end (); ++i)
#define NL puts("")
#define sd(a) scanf("%lf",&a)
#define sdd(a,b) scanf("%lf %lf",&a,&b)
#define sddd(a,b,c) scanf("%lf %lf %lf",&a,&b,&c)
#define SP printf(" ")
#define LL long long int
#define ULL unsigned long long int
#define MOD 1000000007
#define pub(x) push_back(x)
#define pob(x) pop_back(x)
#define mem(ara,value) memset(ara,value,sizeof(ara))
#define INF INT_MAX
#define eps 1e-8
#define checkbit(n, pos) (n & (1<<pos))
#define setbit(n, pos) (n | (1<<pos))
#define para(a,b,ara)\
for(int i=a;i<=b;i++){\
    if(i!=0){printf(" ");}\
    cout<<ara[i];\
}\
printf("\n");
#define pvec(vec)\
for(int i=0;i<vec.size();i++){\
    if(i!=0){printf(" ");}\
    cout<<vec[i];printf("\n");\
}\
printf("\n");
#define ppara(n,m,ara)\
for(int i=0;i<n;i++){\
    for(int j=0;j<m;j++){\
        if(j!=0){printf(" ");}\
        cout<<ara[i][j];\
    }\
    printf("\n");\
}
#define ppstructara(n,m,ara)\
for(int i=0;i<n;i++){\
    printf("%d:\n",i);\
    for(int j=0;j<m;j++){\
        cout<<ara[i][j];printf("\n")\
    }\
}
#define ppvec(n,vec)\
for(int i=0;i<n;i++){\
    printf("%d:",i);\
    for(int j=0;j<vec[i].size();j++){\
        if(j!=0){printf(" ");}\
        cout<<vec[i][j];\
    }\
    printf("\n");\
}
#define ppstructvec(n,vec)\
for(int i=0;i<n;i++){\
    printf("%d:",i);\
    for(int j=0;j<vec[i].size();j++){\
        cout<<vec[i][j];printf("\n");\
    }\
}
#define sara(a,b,ara)\
for(int i=a;i<=b;i++){\
    scanf("%d",&ara[i]);\
}
#define pstructara(a,b,ara)\
for(int i=a;i<=b;i++){\
    cout<<ara[i];NL;\
}
#define pstructvec(vec)\
for(int i=0;i<vec.size();i++){\
    cout<<vec[i];NL;\
}
#define pstructstl(stl,x)\
for(__typeof(stl.begin()) it=stl.begin();it!=stl.end();++it){\
    x=*it;\
    cout<<x;NL;\
}\
NL;
#define pstl(stl)\
for(__typeof(stl.begin()) it=stl.begin();it!=stl.end();++it){\
    if(it!=stl.begin()){SP;}\
    pi(*it);\
}\
NL;
#define ppairvec(vec)\
for(int i=0;i<vec.size();i++){\
    cout<<vec[i].first;SP;cout<<vec[i].second;printf("\n");\
}
#define ppairara(a,b,ara)\
for(int i=a;i<=b;i++){\
    cout<<ara[i].first;SP;cout<<ara[i].second;printf("\n");\
}
#define pppairvec(n,vec)\
for(int i=0;i<n;i++){\
    printf("%d:",i);\
    for(int j=0;j<vec[i].size();j++){\
        cout<<vec[i][j].first;SP;cout<<vec[i][j].second;NL;\
    }\
}
#define pppairara(n,m,ara)\
for(int i=0;i<n;i++){\
    printf("%d:\n",i);\
    for(int j=0;j<m;j++){\
        cout<<ara[i][j].first;printf(" ");cout<<ara[i][j].second;NL;\
    }\
}
//std::ostream & dump(std::ostream &os)const{
//    return os<<"l="<<l
//             <<"r="<<r
//             <<"m="<<m;
//}
//std::ostream & operator<<(ostream &os,seg &s){
//    return s.dump(os);
//}
using namespace std;
//bool status[1010];
//vector <int> prime;
//void siv(){
//	int N=1005,i,j;prime.clear();
//	int sq=sqrt(N);
//	for(i=4;i<=N;i+=2){status[i]=1};
//	for(i=3;i<=sq;i+=2){
//		if(status[i]==0){
//			for(j=i*i;j<=N;j+=i){status[j]=1;}
//		}
//	}
//	status[1]=1;
//    for1(i,N){
//        if(!status[i]){prime.pub(i);}
//    }
//}
int n,k;
int ara[100010];
void input(){
    int i,j;
    sii(n,k);
    for0(i,n){si(ara[i]);}
}
void solve(){
    int i=n,j,cnt=0;
    while(i>0){
        i-=k;
        cnt++;
        if(i==0){break;}
        i++;
    }
    pi(cnt);NL;
}
int main(){
//    freopen("input.txt","r",stdin);
    input();
    solve();
}


Measured memory_bytes: 640