#include<bits/stdc++.h>
#define lint long long
#define st first
#define nd second
#define INF 0x3f3f3f3f

using namespace std;

struct eq{
    long double a, b; // a*f0 + b
    eq(): a(0), b(0){}
    eq(long double a, long double b): a(a), b(b){}
    eq operator+(const eq &oth){
        return eq(a + oth.a, b + oth.b);
    }
    eq operator-(const eq &oth){
        return eq(a - oth.a, b - oth.b);
    }
    eq operator*(long double f){
        return eq(a*f, b*f);
    }
};


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    int n, m, k;
    cin>>n>>m>>k;
    vector<int> a(k);
    for(int &x:a) cin>>x;
    vector<eq> dp(n+m+1);
    vector<eq> suf(n+m+1);

    for(int i = n-1; i>=0; i--){
        suf[i] = suf[i+1];
        if(binary_search(a.begin(), a.end(), i)){
            dp[i] = {1, 0};
        } else {
            dp[i] = (suf[i+1] - suf[i+m+1]) * (1.0/m);
            dp[i].b += 1.0;
        }
        suf[i]  = suf[i] + dp[i];
    }
    if(abs(1.0 - dp[0].a) < 1e-6)
        cout<<"-1\n";
    else
        cout<<fixed<<setprecision(4)<<dp[0].b/(1.0 - dp[0].a)<<"\n";


    return 0;
}

#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;
}
