#include <bits/stdc++.h>

#if MYDEBUG
#include "lib/cp_debug.hpp"
#else
#define DBG(...) ;
#endif
#if __cplusplus <= 201402L
template <typename T>
T gcd(T a, T b) { return ((a % b == 0) ? b : gcd(b, a % b)); }
template <typename T>
T lcm(T a, T b) { return a / gcd(a, b) * b; }
#endif
using LL = long long;
constexpr LL LINF = 334ll << 53;
constexpr int INF = 15 << 26;
constexpr LL MOD = 1E9 + 7;

namespace Problem {
using namespace std;

struct RangeSum {
  using T = double;
  T operator()(const T &a, const T &b) { return a + b; }
  static constexpr T id() { return T(0); };
};

template <typename M>
struct SegmentTree {
  using Val = typename M::T;
  M calc;
  vector<Val> data;
  int n;
  explicit SegmentTree(int size) {
    n = 1;
    while (n < size) n <<= 1;
    data = vector<Val>(n << 1, calc.id());
  }
  explicit SegmentTree(vector<Val> &v) {
    n = 1;
    while (n < v.size()) n <<= 1;
    data = vector<Val>(n << 1);
    copy(v.begin(), v.end(), data.begin() + n);
    for (int i = n + (int)v.size(); i < 2 * n; i++) data[i] = calc.id();
    for (int i = n - 1; i > 0; --i) {
      data[i] = calc(data[i * 2], data[i * 2 + 1]);
    }
  }
  void update(int i, Val v) {
    int k = i + n;
    data[k] = v;
    for (k >>= 1; k >= 1; k >>= 1) data[k] = calc(data[k * 2], data[k * 2 + 1]);
  }
  Val query(int l, int r) {  //get[a,b)
    Val resl = calc.id(), resr = calc.id();
    for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
      if (l & 1) resl = calc(resl, data[l++]);
      if (r & 1) resr = calc(data[r - 1], resr);
    }
    return calc(resl, resr);
  }
};

class Solver2 {
 public:
  int n, m;
  vector<int> is_return;
  SegmentTree<RangeSum> a_0, a_x;
  Solver2(LL n, LL m) : n(n), m(m), is_return(n + 1), a_0(n + 1), a_x(n + 1){};

  void solve() {
    int k;
    cin >> k;
    for (int i = 0; i < k; ++i) {
      int tmp;
      cin >> tmp;
      is_return[tmp] = 1;
    }
    for (int i = 0, j = 0; i <= n; ++i) {
      if (is_return[i]) {
        j++;
        if (j == m) {
          cout << -1 << endl;
          return;
        }
      } else {
        j = 0;
      }
    }
    a_0.update(n, 0);
    a_x.update(n, 0);
    for (int i = n - 1; i > 0; --i) {
      if (is_return[i]) {
        a_0.update(i, 0);
        a_x.update(i, 1);
      } else {
        double sum_0 = a_0.query(i + 1, min(i + m, n) + 1);
        double sum_x = a_x.query(i + 1, min(i + m, n) + 1);
        a_0.update(i, sum_0 / m + 1.0);
        a_x.update(i, sum_x / m);
        if (i > 99900) DBG(i, m - (n - i), sum_0 / m + 1.0, sum_x / m)
      }
    }
    double sum_0 = a_0.query(1, min(m, n) + 1);
    double sum_x = a_x.query(1, min(m, n) + 1);
    DBG(sum_0 / m + 1.0, sum_x / m)
    double ans = (sum_0 / m + 1.0) / (1.0 - sum_x / m);
    cout << ans << endl;
  }
};
}  // namespace Problem

int main() {
  std::cin.tie(0);
  std::ios_base::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(12);
  long long n = 0, m;
  std::cin >> n >> m;

  Problem::Solver2 sol(n, m);
  sol.solve();
  return 0;
}

#include<bits/stdc++.h>
#define Pair pair<ll,int>
#define ar array
#define ll long long
#define ull unsigned long long 
#define pb push_back

using namespace std ;
const ll LLMAX = 9223372036854775807;
const int INF = 1e9;

struct Exp{
    long double a, b;
};

Exp operator +(Exp x, Exp y){
    return {x.a+y.a, x.b+y.b};
}

Exp operator -(Exp x, Exp y){
    return {x.a-y.a, x.b-y.b};
}
Exp operator /(Exp x, long double d){
    return {x.a/d, x.b/d};
}

bool mark[200005];
Exp dp[200005];
Exp sum[200005];
int n, m, k;

int main(){
    cin>>n>>m>>k;
    vector<int> kk;
    for(int i=0; i<k; i++){
        int a;
        cin>>a;
        kk.pb(a);
        mark[a] = true;
    }

    sort(kk.begin(), kk.end());
    if(k>0){
        int consec = 1;
        int max_consec = 1;
        for(int i=1; i<kk.size(); i++){
            if(kk[i]-kk[i-1]!=1){
                consec = 1;
            }
            else
                consec++;
            max_consec = max(max_consec, consec);
        }
        if(max_consec>=m){
            cout<<-1<<endl;
            return 0;
        }
    }

    sum[n] = {0,0};
    for(int i=n-1; i>=0; i--){
        if(!mark[i])
            dp[i] = (sum[i+1]-sum[i+m+1])/m + (Exp){0,1};
        else
            dp[i] = {1,0};
        sum[i] = sum[i+1]+dp[i];
    }

    long double res = dp[0].b / (1-dp[0].a);
    cout<<fixed<<setprecision(4)<<res<<endl;
}
