//Never stop trying
#include "bits/stdc++.h"
using namespace std;
#define boost ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)

typedef long long ll;
#define int ll
typedef string str;
typedef long double ld;
typedef pair<int, int> pi;
#define fi first
#define se second
typedef vector<int> vi;
typedef vector<pi> vpi;
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define endl "\n"
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)

const int MOD = 1e9 + 7; //998244353
const ll INF = 1e18;
const int MX = 2e5 + 10;
const int nx[4] = {0, 0, 1, -1}, ny[4] = {1, -1, 0, 0}; //right left down up

template<class T> using V = vector<T>;
template<class T> bool ckmin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
ll cdiv(ll a, ll b) { return a / b + ((a ^ b) > 0 && a % b); } // divide a by b rounded up
//constexpr int log2(int x) { return 31 - __builtin_clz(x); } // floor(log2(x))

mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
//mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
ll random(ll a, ll b){
    return a + rng() % (b - a + 1);
}
#ifndef LOCAL  
#define cerr if(false) cerr
#endif
#define dbg(x) cerr << #x << " : " << x << endl; 
#define dbgs(x,y) cerr << #x << " : " << x << " / " << #y << " : " << y << endl;
#define dbgv(v) cerr << #v << " : " << "[ "; for(auto it : v) cerr << it << ' '; cerr << ']' << endl;
#define here() cerr << "here" << endl;

void IO() {
#ifdef LOCAL
    freopen("input.txt", "r", stdin); 
    freopen("output.txt", "w", stdout);
#endif
}
/////////////////////////ONLY CLEAN CODES ALLOWED/////////////////////////


int32_t main() {
    boost; IO();

    int N,M; cin>>N>>M;
    vi bad(N,0);
    int K; cin>>K;
    FOR(i,0,K){
    	int x; cin>>x;
    	bad[x]=1;
    }
    int cnt=0;
    FOR(i,0,N){
    	if(bad[i]) cnt++;
    	else cnt=0;
    	if(cnt>=M){
    		cout << -1 << endl;
    		return 0;
    	}
    }
    
    ld cur_a=0,cur_b=0; //for the last M elements
    V<ld>a(N,0),b(N,0);
    ROF(i,0,N){
    	if(bad[i]){
    		a[i]=1;
    	}
    	else{
    		a[i]=cur_a/M;
    		b[i]=cur_b/M+1;
    	}
    	cur_a+=a[i];
    	cur_b+=b[i];
    	if(i+M<N){
    		cur_a-=a[i+M];
    		cur_b-=b[i+M];
    	}
    }

    cout << fixed << setprecision(7) << -b[0]/(a[0]-1) << endl;

    return 0;
}
//Change your approach 
#include <bits/stdc++.h>

#define fi first
#define se second
#define mp make_pair
#define lowbit(x) ((x) & -(x))

using namespace std;
using ll = long long;
using pii = pair<int, int>;

constexpr double eps = 1e-8;
constexpr int NINF = 0xc0c0c0c0;
constexpr int INF = 0x3f3f3f3f;
constexpr ll mod = 1e9 + 7;
constexpr ll N = 1e6 + 5;

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

    ll B, C;
    cin >> B >> C;
    if (B > 0) {
        ll L = B - C / 2;
        ll R = (C % 2 == 0 ? -(L + 1): -L);
        ll l = -B - (C - 1) / 2;
        ll r = C % 2 == 0 ? -l : -(l + 1);
        r = max(r, B);
        ll res = B - L + 1 + R - max(l, B + 1) + 1;
        if (L <= 0) {
            res = r - l + 1;
        } else {
            res = R - l + 1 + r - L + 1;
        }
        cout << res << '\n';
    } else {
        ll L = B - C / 2;
        ll R = (C % 2 == 0 ? -(L + 1): -L);
        ll l = max(-B - (C - 1) / 2, B + 1);
        ll res = B - L + 1 + R - l + 1;
        cout << res << '\n';
    }

    return 0;
}