#include<bits/stdc++.h>
#define ll unsigned long long
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
using namespace std;
const int N=2e5+10;
const double eps=1e-8;
bool ban[N];
double k[N],b[N];
int main()
{
    int n,m,K;
    scanf("%d%d%d",&n,&m,&K);
    for(int i=1;i<=K;i++){int a;scanf("%d",&a),ban[a]=1;}
    double km=0,bm=0;
    for(int i=n-1;~i;i--)
    {
        if(ban[i])k[i]=1,b[i]=0;
        else k[i]=km/m,b[i]=bm/m+1;
        km+=k[i]-k[i+m],bm+=b[i]-b[i+m];
    }
    if(fabs(1-k[0])<eps)puts("-1");
    else printf("%.10lf\n",b[0]/(1-k[0]));
	return 0;
}

#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
// #include<ext/pb_ds/assoc_container.hpp>
// #include<ext/pb_ds/tree_policy.hpp>
// #include<ext/pb_ds/tag_and_trait.hpp>
// using namespace __gnu_pbds;
// #include<boost/multiprecision/cpp_int.hpp>
// namespace multiprecisioninteger = boost::multiprecision;
// using cint=multiprecisioninteger::cpp_int;
using namespace std;
using ll=long long;
#define double long double
using datas=pair<ll,ll>;
using ddatas=pair<double,double>;
using tdata=pair<ll,datas>;
using vec=vector<ll>;
using mat=vector<vec>;
using pvec=vector<datas>;
using pmat=vector<pvec>;
// using llset=tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>;
#define For(i,a,b) for(i=a;i<(ll)b;++i)
#define bFor(i,b,a) for(i=b,--i;i>=(ll)a;--i)
#define rep(i,N) For(i,0,N)
#define rep1(i,N) For(i,1,N)
#define brep(i,N) bFor(i,N,0)
#define brep1(i,N) bFor(i,N,1)
#define all(v) (v).begin(),(v).end()
#define allr(v) (v).rbegin(),(v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define uniq(v) vsort(v);(v).erase(unique(all(v)),(v).end())
#define endl "\n"
#define eb emplace_back
#define print(x) cout<<x<<endl
#define printyes print("Yes")
#define printno print("No")
#define printYES print("YES")
#define printNO print("NO")
#define output(v) do{bool f=0;for(auto outi:v){cout<<(f?" ":"")<<outi;f=1;}cout<<endl;}while(0)
#define matoutput(v) do{for(auto outimat:v)output(outimat);}while(0)
constexpr ll mod=1000000007;
// constexpr ll mod=998244353;
constexpr ll inf=1LL<<60;
constexpr double eps=1e-9;
const double PI=acos(-1);
template<class T,class E> ostream& operator<<(ostream& os,const pair<T,E>& p){return os<<"("<<p.first<<","<<p.second<<")";}
template<class T> ostream& operator<<(ostream& os,const vector<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> ostream& operator<<(ostream& os,const set<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> inline bool chmax(T& a,T b){bool x=a<b;if(x)a=b;return x;}
template<class T> inline bool chmin(T& a,T b){bool x=a>b;if(x)a=b;return x;}
#ifdef DEBUG
void debugg(){cout<<endl;}
template<class T,class... Args>void debugg(const T& x,const Args&... args){cout<<" "<<x;debugg(args...);}
#define debug(...) cout<<__LINE__<<" ["<<#__VA_ARGS__<<"]:",debugg(__VA_ARGS__)
#else
#define debug(...) (void(0))
#endif

void startupcpp(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout<<fixed<<setprecision(15);
}

ll N,M,K;
int main(){
  startupcpp();
  // int codeforces;cin>>codeforces;while(codeforces--){
  ll i,j;
  cin>>N>>M>>K;
  vector<double> sumdpx(N+1,0),sumdpa(N+1,0);
  rep(i,K){
    cin>>j;
    sumdpx[j]=1;
  }
  //x=ax+b のa,bを求める
  brep(i,N){
    if(sumdpx[i]){
      sumdpx[i]+=sumdpx[i+1];
      sumdpa[i]+=sumdpa[i+1];
      continue;
    }
    sumdpx[i]=(sumdpx[i+1]-sumdpx[min(i+M+1,N)])/M+sumdpx[i+1];
    sumdpa[i]=(sumdpa[i+1]-sumdpa[min(i+M+1,N)])/M+1+sumdpa[i+1];
  }
  sumdpx[0]-=sumdpx[1];
  sumdpa[0]-=sumdpa[1];
  double ans=sumdpa[0]/abs(1-sumdpx[0]);
  if(ans>1e18){print(-1);return 0;}
  print(ans);
}