#include <vector>
#include <stack>
#include <queue>
#include <list>
#include <bitset>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <algorithm>
#include <numeric>
#include <iostream>
#include <iomanip>
#include <string>
#include <chrono>
#include <random>
#include <cmath>
#include <cassert>
#include <climits>
#include <cstring>
#include <cstdlib>
#include <functional>
#include <sstream>

using namespace std;


int main(int argc, char** argv) {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(12);

    int n, m, K;
    cin >> n >> m >> K;

    vector<bool> bad(n + 1, false);
    vector<int> A(K);
    for (int i = 0; i < K; ++i) {
        cin >> A[i];
        bad[A[i]] = true;
        A[i] -= i;
    }

    for (int i = 0; i < K; ++i) {
        int len = 1;
        while (i + 1 < K && A[i] == A[i + 1]) {
            ++i;
            ++len;
        }
        if (len >= m) {
            cout << -1 << '\n';
            return 0;
        }
    }


    vector<long double> dp(n + 1, 0);
    auto calc = [&](long double x) {
        fill(dp.begin(), dp.end(), 0);
        long double sum = 0;
        for (int i = n - 1; i >= 0; --i) {
            if (bad[i]) {
                dp[i] = x;
            } else {
                dp[i] = sum / m  + 1.0;
            }
            sum += dp[i];
            if (i + m <= n) {
                sum -= dp[i + m];
            }
        }

        return dp[0];
    };
    long double lo = 0, hi = 1e20;

    int rnd = 128;
    while (rnd-- > 0) {
        long double mi = (lo + hi) / 2;

        auto ans = calc(mi);
        if (ans < mi) {
            hi = mi;
        } else {
            lo = mi;
        }
    }

    cout << lo << '\n';

    return 0;
}
// abc189_f
#include <bits/stdc++.h>
#ifdef LOCAL
#include "../cxx-prettyprint/prettyprint.hpp"
#endif
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<long double, long double> P;

#define REP(i, n) for (int (i) = 0 ; (i) < (int)(n) ; ++(i))
#define REPN(i, m, n) for (int (i) = m ; (i) < (int)(n) ; ++(i))
#define REP_REV(i, n) for (int (i) = (int)(n) - 1 ; (i) >= 0 ; --(i))
#define REPN_REV(i, m, n) for (int (i) = (int)(n) - 1 ; (i) >= m ; --(i))
#define ALL(x) x.begin(), x.end()

#define INF (ll)(1e9)
#define MOD (1000000007)

#define print2D(h, w, arr) REP(i, h) { REP(j, w) cout << arr[i][j] << " "; cout << endl; }
#define print_line(vec, n) {for(int idx=0;idx<(n-1);idx++) cout << (vec)[idx] << " "; cout << (vec)[(n)-1] << endl;}
template<class T> void print(const T& x){cout << x << "\n";}
template<class T, class... A> void print(const T& first, const A&... rest) { cout << first << " "; print(rest...); }
struct PreMain {PreMain(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(20);}} premain;

int main() {
#ifdef LOCAL
    ifstream in("../arg.txt"); cin.rdbuf(in.rdbuf());
#endif

    int N, M, K;
    cin >> N >> M >> K;
    vector<bool> A(N);
    REP(i, K) {
        int a; cin >> a; a;
        A[a] = true;
    }

    vector<P> V(N+M);
    P cumsum = {0, 0};
    REP_REV(i, N){
        if (A[i]){
            V[i] = {1, 0};
        } else {
            V[i] = {cumsum.first / M, cumsum.second / M + 1};
        }
        cumsum.first = {cumsum.first - V[i+M].first + V[i].first};
        cumsum.second = {cumsum.second - V[i+M].second + V[i].second};

//        print(i, cumsum);
    }

//    REP(i, N+M) print(V[i]);

    if (abs(V[0].first - 1) < 1e-6){
        print(-1);
    } else {
        long double ans = V[0].second / (1 - V[0].first);
        print(ans);
    }


    return 0;
}
