#include <bits/stdc++.h>

using namespace std;

typedef long double ld;

const int N = (int) 2e5 + 7;
int n;
int m;
int k;
int x[N];
bool is[N];
ld dp[N];

ld algo(ld dp0) {
  ld sum = 0;
  for (int i = n - 1; i >= 0; i--) {
    if (is[i]) {
      dp[i] = dp0;
    } else {
      dp[i] = 1 + sum / (ld) m;
    }
    sum += dp[i];
    sum -= dp[i + m];
  }
  return dp[0];
 /// cout << dp0 << " " << dp[0] << "\n";
}

bool ok(ld dp0) {
  return dp0 - algo(dp0) < 1e-14;
}

const ld INF = (ld) 1e12;

signed main() {
  ios::sync_with_stdio(0); cin.tie(0);

 /// freopen ("input", "r", stdin);

  cin >> n >> m >> k;
  for (int i = 1; i <= k; i++) {
    int x;
    cin >> x;
    is[x] = 1;
  }
  ld l = 0, r = INF;
  if (ok(r)) {
    cout << "-1\n";
    return 0;
  }
  for (int i = 1; i <= 100; i++) {
    ld m = (l + r) * 0.5;
    if (ok(m)) {
      l = m;
    } else {
      r = m;
    }
  }
  cout << fixed << setprecision(3) << l << "\n";
  return 0;
  ld x = 0;
  while (1) {
    algo(x);
    ld y = dp[0];
    if (fabs(x - y) < 1e-14) {
      break;
    }
    x = y;
  }
  cout << fixed << setprecision(6) << x << "\n";
}

/*#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
*/

#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 
#define ordered_set tree<pair<ll,ll>, null_type,less<pair<ll,ll>>, rb_tree_tag,tree_order_statistics_node_update> 
*/
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
#define rep(i,a,b) for(ll i=a;i<=b;++i)
#define rrep(i,a,b) for(ll i=a;i>=b;--i)
#define FOR(i,n)  for(ll i=0;i<n;i++)
#define pb push_back
#define mp make_pair
#define PI 3.14159265358979323846
#define fi first
#define se second
#define all(x)  x.begin(),x.end()
#define IOS ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
 
const ll INF = 1e18+7;
const ll mod = 998244353;
const ll MAXN = 1e5+10;
 
ll poww(ll a, ll b) {    
    if(b<0) return 0LL;
    ll ans = 1;
    while (b) {
        if (b & 1)ans = ans * a;
        a = a * a;
        b >>= 1;
    }   
    return ans;
}
ll binpow(ll a, ll b)
{
    if (b < 0) return 0LL;
    if (a <= 0)return 0LL;
    a %= mod;
    
    ll ans = 1LL;
    while (b) {
        if (b & 1)ans = ans * a % mod;
        a = a * a % mod;
        b >>= 1;
    }
    return ans;
}
ll modinv(ll n) {
    return binpow(n, mod - 2);
}

void solve() {
    ll n,m,k; cin>>n>>m>>k;
    vector<ll> pos(n+1,0);
    FOR(i,k) {
        ll x; cin>>x;
        pos[x] = 1;
    }
    ll ct = 0, flg = 0;
    for(ll i=1; i<n; i++) {
        if(pos[i]==0) {
            if(ct>=m) flg = 1;
            ct = 0; 
        }
        else ct++;
    }
    if(ct>=m) flg = 1;
    if(flg) { cout<<-1; return; }

    
    vector<ld> a(n+1), b(n+1);
    FOR(i,n+1) a[i] = b[i] = 0.0;
    
    ld r1=0.0,r2=0.0;
    for(ll i=n-1; i>=0; i--) {
        if(pos[i]) {
            a[i] = 1.0; b[i] = 0.0;
        }
        else {
            ld x = 0.0, y = 0.0;
            x = r1; y = r2;
            x /= (m*1.0); y /= (m*1.0); y += 1.0;
            a[i] = x; b[i] = y;
        }
        r1 += a[i]; r2 += b[i];
        if(i+m<=n) {
            r1 -= a[i+m]; r2 -= b[i+m];
        }
    }

    ld ans = b[0] / (1-a[0]);
    cout<<ans;
}

int main() {
    IOS; 
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
    #endif
        
    //ll no_of_test_cases; cin>>no_of_test_cases; 
    ll no_of_test_cases = 1;
    cout<<setprecision(10);
    
    for(ll i=1; i<=no_of_test_cases; i++) {
        solve();
    } 
    return 0; 
}