#include<iostream>

#include<cstring>

int hol[];

using namespace std;

int main()

{

    int n,k,c;

    while(cin>>n>>k)

    {

        memset(hol,,sizeof(hol));

        cin>>c;

        int sum=;

        sum+=c;

        if(c!=)

        {

            cin>>hol[];

            if(hol[]->=k)

            {

                sum+=(hol[]-)/k;

            }

            for(int i=;i<c;i++)

            {

                cin>>hol[i];

                if(hol[i]-hol[i-]->=k)

                {

                    sum+=(hol[i]-hol[i-]-)/k;

                }

            }

            if(n-hol[c-]>=k)

            {

                sum+=(n-hol[c-])/k;

            }

        }

        else   //没有假日的情况

        {

                sum+=n/k;

        }

        cout<<sum<<endl;

    } 

    return ;

}