//@formatter:off
#include<bits/stdc++.h>
#define overload4(_1,_2,_3,_4,name,...) name
#define rep1(i,n) for (ll i = 0; i < ll(n); ++i)
#define rep2(i,s,n) for (ll i = ll(s); i < ll(n); ++i)
#define rep3(i,s,n,d) for(ll i = ll(s); i < ll(n); i+=d)
#define rep(...) overload4(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep(i,n) for (ll i = ll(n)-1; i >= 0; i--)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define pb push_back
#define eb emplace_back
#ifdef __LOCAL
#define debug(...) { cout << #__VA_ARGS__; cout << ": "; print(__VA_ARGS__); cout << flush; }
#else
#define debug(...) void(0)
#endif
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<int,int>;
using LP = pair<ll,ll>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vd = vector<double>;
using vvd = vector<vector<double>>;
using vs = vector<string>;
using vc = vector<char>;
using vvc = vector<vector<char>>;
using vb = vector<bool>;
using vvb = vector<vector<bool>>;
using vp = vector<P>;
using vvp = vector<vector<P>>;
template<class S,class T> istream& operator>>(istream &is,pair<S,T> &p) { return is >> p.first >> p.second; }
template<class S,class T> ostream& operator<<(ostream &os,const pair<S,T> &p) { return os<<'{'<<p.first<<","<<p.second<<'}'; }
template<class T> istream& operator>>(istream &is,vector<T> &v) { for(T &t:v){is>>t;} return is; }
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) { os<<'[';rep(i,v.size())os<<v[i]<<(i==int(v.size()-1)?"":","); return os<<']'; }
template<class T> bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;}
template<class T> bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;}
void scan(){}
template <class Head, class... Tail> void scan(Head& head, Tail&... tail){ cin >> head; scan(tail...); }
template<class T> void print(const T& t){ cout << t << '\n'; }
template <class Head, class... Tail> void print(const Head& head, const Tail&... tail){ cout<<head<<' '; print(tail...); }
template<class... T> void fin(const T&... a) { print(a...); exit(0); }
const string yes[] = {"no","yes"};
const string Yes[] = {"No","Yes"};
const string YES[] = {"NO","YES"};
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

double A[200010];
double B[200010];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    INT(n, m, k);
    vi a(k);
    cin >> a;
    if (m <= k) {
        rep(i, k - m + 1) {
            bool ok = true;
            rep(j, m - 1) ok &= a[i + j] + 1 == a[i + j + 1];
            if (ok) fin(-1);
        }
    }
    cout << fixed << setprecision(10);
    rrep(i, n) {
        bool ok = true;
        rep(j, k) if (i == a[j]) ok = false;
        double na, nb;
        if (ok) {
            na = (A[i + 1] - A[i + m + 1]) / m;
            nb = (B[i + 1] - B[i + m + 1]) / m + 1;
        } else {
            na = 1, nb = 0;
        }
        A[i] = A[i + 1] + na;
        B[i] = B[i + 1] + nb;
    }
    double na = A[0] - A[1], nb = B[0] - B[1];
    // X = na * X + nb
    print(nb / (1 - na));
}

#include <complex>
#include <iostream>
#include <vector>
using namespace std;

#define rep(i, n) for(int i = 0; i < (int)(n); i++)

int main() {
    int N, M, K;
    cin >> N >> M >> K;

    vector< complex< double > > means(N);
    rep(i, K) {
        int a;
        cin >> a;
        means[a].imag(1);
    }

    complex< double > acc = 0;
    int l = 0;
    int suc_mass = 0;
    for(int k = N - 1; k >= 0; k--) {
        if(means[k].imag() == 0) {
            means[k] += M;
            means[k] += acc;
            means[k] /= M;
            suc_mass = 0;
        } else {
            ++suc_mass;
            if(suc_mass >= M)
                break;
        }
        acc += means[k];
        ++l;
        if(l > M) {
            acc -= means[k + M];
            --l;
        }
    }
    // cout << means[0].real() << ", " << means[0].imag() << endl;
    if(suc_mass >= M) {
        cout << -1 << endl;
    } else {
        double res = means[0].real();
        res /= (1.0 - means[0].imag());
        cout << res << endl;
    }
}