//clear adj and visited vector declared globally after each test case
//check for long long overflow
//while adding and subs check if mod becomes -ve
//while using an integer directly in a builtin function add ll
//Mod wale question mein last mein if dalo ie. Ans<0 then ans+=mod;
//Dont keep array name as size or any other key word
//Incase of close mle change language to c++17 or c++14
/**#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops”)**/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#define int long long
#define IOS std::ios::sync_with_stdio(false); cin.tie(NULL);cout.tie(NULL);cout.precision(dbl::max_digits10);
#define pb push_back
#define mod 1000000007ll //998244353ll
#define lld long double
#define mii map<int, int>
#define mci map<char, int>
#define msi map<string, int>
#define pii pair<int, int>
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define rep(i,x,y) for(int i=x; i<y; i++)
#define fill(a,b) memset(a, b, sizeof(a))
#define vi vector<int>
#define setbits(x) __builtin_popcountll(x)
#define print2d(dp,n,m) for(int i=0;i<=n;i++){for(int j=0;j<=m;j++)cout<<dp[i][j]<<" ";cout<<"\n";}
typedef std::numeric_limits< double > dbl;
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;
//member functions :
//1. order_of_key(k) : number of elements strictly lesser than k
//2. find_by_order(k) : k-th element in the set
const long long N=100005, INF=2000000000000000000;
lld pi=3.1415926535897932;
int lcm(int a, int b)
{
int g=__gcd(a, b);
return a/g*b;
}
int power(int a, int b, int p)
{
if(a==0)
return 0;
int res=1;
a%=p;
while(b>0)
{
if(b&1)
res=(res*a)%p;
b>>=1;
a=(a*a)%p;
}
return res;
}
lld dp[N], suf[N], n, m, dp2[N], inv, ans=0.0;
void fun(int pos)
{
if(pos==n)
return;
fun(pos+1);
int rg=min(n, pos+m+1);
if(suf[pos]==suf[pos+1])
{
dp[pos]=((((dp[pos+1]-dp[rg]))/m)+1.0);
dp2[pos]=((dp2[pos+1]-dp2[rg]+suf[pos+1]-suf[rg])/m);
}
if(pos==0)
{
dp2[pos]=(1.0-dp2[pos]);
if(dp2[pos]==0)
ans=-1;
else
ans=(dp[pos]/dp2[pos]);
}
dp[pos]=(dp[pos]+dp[pos+1]);
dp2[pos]=(dp2[pos]+dp2[pos+1]);
}
int32_t main()
{
IOS;
int k;
cin>>n>>m>>k;
inv=power(m, mod-2, mod);
rep(i,0,N)
{
dp[i]=dp2[i]=suf[i]=0.0;
}
rep(i,0,k)
{
int a;
cin>>a;
suf[a]+=1.0;
}
for(int i=n-1;i>=0;i--)
suf[i]+=suf[i+1];
fun(0);
lld eps=1e-9;
if(ans<=eps)
cout<<-1;
else
cout<<ans;
}