
// --------------------<optimizations>--------------------
#pragma GCC optimize("O3")

//(UNCOMMENT WHEN HAVING LOTS OF RECURSIONS)
#pragma comment(linker, "/stack:200000000")
//(UNCOMMENT WHEN TRYING TO BRUTEFORCE WITH A LOT OF LOOPS)
#pragma GCC optimize("unroll-loops")
// -------------------</optimizations>--------------------
#include <iostream>
#include <bitset>
#include <algorithm>        // sort()
#include <vector>           // for vectors
#include <map>              // for ordered map
#include <unordered_map>
#include <list>             // for list
#include <iomanip>          // for setprecision()
#include <cmath>            // for pow(), sqrt(), log(), log10(), ....
#include <cstring>          // for memset()
#include <unordered_set>    // for unordered_set
#include <climits>          // for INT_MAX & INT_MIN
#include <set>              // for ordered set

#define C ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
using namespace std;
typedef long long ll;
#define INF INT_MAX
#define NINF INT_MIN
#define MOD 1000000007
const double pi=acos(-1); // value of pi


bool isVowel(char ch) {
    if(ch=='a' || ch=='e' || ch=='i' || ch=='o' || ch=='u') {
        return 1;
    }
    return 0;
}

bool compare(pair<ll, ll> a, pair<ll, ll> b) {
    return a.second<b.second;
}


bool isPrime(ll n) {
    if(n%2==0 && n!=2) {
        return false;
    }
    for(ll i=3; i*i<=n; i+=2) {
        if(n%i==0) {
            return false;
        }
    }
    return true;
}

ll binary_exp(ll x, ll n, ll mod)
{
    if (n == 0) {
        return 1;
    } else if (n == 1) {
        return x % mod;
    } else {
        ll temp = binary_exp(x, n / 2, mod);
        temp = (temp * temp) % mod;

        if (n % 2 == 0) {
            return temp;
        } else {
            return ((x % mod) * temp) % mod;
        }
    }
}

map<ll, ll> a, b;
// map<ll, ll> m;

int main() {
    C
//     freopen("input.txt","r",stdin);
// 	freopen("output.txt","w",stdout);
    int t=1;
    // cin >> t;
    while(t--) {
        ll n, a, b;
        cin >> n >> a >> b;
        cout << n-a+b << endl;
    }
}

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int inf=0x3f3f3f3f;
const ll INF=0x3f3f3f3f3f3f3f;
const double pi=3.1415926535897932384626;
inline ll read(){
	ll x=0,f=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-') f=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=(x<<1)+(x<<3)+(ch^48);
		ch=getchar();
	}
	return x*f;
}
const int maxn=1e5+5;
int n,m,k,x,vis[maxn];
double dp[maxn][2],sum[maxn][2];
int main(){
	n=read(),m=read(),k=read();
	for(int i=1;i<=k;++i){
		x=read();
		vis[x]=1;
	}
	dp[n][0]=dp[n][1]=0;
	for(int i=n-1;i>=0;--i){
		if(vis[i]){
			dp[i][0]=1,dp[i][1]=0;
		}
		else{
			dp[i][0]=(sum[i+1][0]-sum[i+m+1][0])/m;
			dp[i][1]=(sum[i+1][1]-sum[i+m+1][1])/m+1.0;
		}
		sum[i][0]=sum[i+1][0]+dp[i][0];
		sum[i][1]=sum[i+1][1]+dp[i][1];
	}
	if(fabs(dp[0][0]-1.0)<1e-6) printf("-1\n");
	else printf("%.4f\n",dp[0][1]/(1.0-dp[0][0]));
	return 0;
}