#include <bits/stdc++.h>
typedef long long ll ;
#define int ll
const int MAX=2e5+10;
const int modn=1e9+7;
const int INF=0x3f3f3f3f3f3f;
#define endl '\n'
#define cwk     freopen("D:\\workplace\\CLion\\in.in","r",stdin),freopen("D:\\workplace\\CLion\\out.out","w",stdout)
using namespace std;
int n,m,k,x;
bool vis[MAX];
double a[MAX],b[MAX];
bool check(){
    int cnt=0;
    for(int i=1;i<=n;i++){
        if(vis[i]) cnt++;
        else{
            if(cnt>=m) return false;
            cnt=0;
        }
    }
    return true;
}
void solve(){
    cin>>n>>m>>k;
    while(k--){
        cin>>x;
        vis[x]=true;
    }
    if(!check()){
        cout<<"-1\n";
        return;
    }
    double sumb=0;
    double suma=0;
    for(int i=n-1;i>=0;i--){
        if(vis[i]){
            a[i]=1;
            b[i]=0;
        }else{
            a[i]=suma*1.0/m;
            b[i]=sumb*1.0/m+1;
        }
        sumb+=b[i]-b[i+m];
        suma+=a[i]-a[i+m];
    }
    printf("%.4f",b[0]/(1-a[0]));
}
signed main() {
    ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
    //cwk;
    int _=1;
    //cin>>_;
    while(_--){
        solve();
    }
    return 0;
}
#include <bits/stdc++.h>
#define FOR(i, a, n) for(ll i = (ll)a; i < (ll)n; i++)
#define FORR(i, n) for(ll i = (ll)n - 1LL; i >= 0LL; i--)
#define rep(i, n) FOR(i, 0, n)
#define ALL(x) begin(x), end(x)
using namespace std;
using ll = long long;
constexpr ll Mod = 998244353;
constexpr ll mod = 1e9 + 7;
constexpr ll inf = 1LL << 60;
const double PI = acos(-1);
template <typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {
    return a < b && (a = b, true);
}
template <typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {
    return a > b && (a = b, true);
}
/*-------------------------------------------*/

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n, m, k;
    cin >> n >> m >> k;

    bool a[100009]{};
    rep(i, k) {
        int x;
        cin >> x;
        a[x] = true;
    }

    vector<complex<double>> dp(n + 1);
    dp[n] = 0;
    complex<double> sum = 0;

    FORR(i, n) {
        if(a[i])
            dp[i] = complex<double>(0, 1);
        else
            dp[i] = sum / double(m) + 1.;
        sum += dp[i];
        if(i + m <= n) sum -= dp[i + m];
    }

    if(dp[0].imag() + 1e-8 > 1) {
        cout << -1 << endl;
        return 0;
    }

    cout << fixed << setprecision(10);

    cout << dp[0].real() / (1 - dp[0].imag()) << endl;

    return 0;
}