#include <iostream>
#include <cstdio>
#include <vector>
#include <unordered_set>

using namespace std;


class Solution{

public:
    double solve(int N, int M, int K, const vector<int>& A){

        unordered_set<int> Aset(A.begin(), A.end());
        vector<double> a(N + M, 0.0), b(N + M, 1.0);
        double asum = 0.0, bsum = M;

        for(int i = N - 1; i >= 0; i --){
            if(Aset.count(i))
                a[i] = 1.0, b[i] = 0.0;
            else
                a[i] = asum / M, b[i] = bsum / M + 1.0;

            asum -= a[i + M], asum += a[i];
            bsum -= b[i + M], bsum += b[i];
        }

        if(abs(a[0] - 1) < 1e-8) return -1;
        return - b[0] / (a[0] - 1.0) - 1.0;
    }
};

int main() {

    int N, M, K; cin >> N >> M >> K;

    vector<int> A(K);
    for(int i = 0; i < K; i ++) cin >> A[i];

    double res = Solution().solve(N, M, K, A);
    if(res < 0) cout << -1 << endl;
    else printf("%.4lf", res);

    return 0;
}

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (ll i=(ll)from; i<(ll)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (ll i=(ll)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const ll INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(ll x=pow(10,7)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { ll d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll modpow(ll x, ll p) { ll result = 1, now = 1, pm = x; while (now<=p) { if (p&now) { result = result * pm % MOD; } now*=2; pm = pm*pm % MOD; } return result; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }


int main() {
  ll N, M, K; cin >> N >> M >> K;
  ll A[K]; rep(i,K) cin >> A[i];

  double dp[N+1][2] = {}; rep(i,K) dp[A[i]][1] = 1;
  double rsum[N+1][2] = {};
  per(i,N) {
    if (dp[i][1]!=1) {
      dp[i][0] = (i+1+M>N?rsum[i+1][0]:rsum[i+1][0]-rsum[i+1+M][0])/M + 1;
      dp[i][1] = (i+1+M>N?rsum[i+1][1]:rsum[i+1][1]-rsum[i+1+M][1])/M;
    }
    rsum[i][0] = rsum[i+1][0] + dp[i][0];
    rsum[i][1] = rsum[i+1][1] + dp[i][1];
  }
  // rep(i,N+1) debug(dp[i],dp[i]+2);
  // rep(i,N+1) debug(rsum[i],rsum[i]+2);

  double result = dp[0][1]==1 ? -1 : dp[0][0]/(1-dp[0][1]);
  cout << setprecision(10) << result << endl;
  return 0;
}