// clang-format off
#include <cassert>
#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
#include <iomanip>

#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (b)-1; i >= (a); i--)
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i,n) for(int i = 1; i <= (n); i++)
#define rrep(i, n) for(int i = (n)-1; i >= 0; i--)

#define pb push_back
#define mp make_pair
#define fst first
#define snd second
#define show(x) cout << #x << " = " << x << endl
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
#define pii pair<int, int>
#define vi vector<int>

using namespace std;
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T>& p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T> ostream& operator<<(ostream& o,const vector<T>& vc){o<<"sz = "<<vc.size()<<endl<<"[";for(const T& v:vc) o<<v<<",";o<<"]";return o;}
typedef long long ll;

ll N;
ll S;
ll w;
#define MAX 77777

// s-50S <= 50S/(0.8+0.2R) < s-50S+1
// 50S/(s-50S+1) < 0.8+0.2R <= 50S/(s-50S)
// 250S/(s-50S+1)-4 < R <= 250S/(s-50S) - 4
ll calc(ll score)
{
    if(score < (50*S+(250*S)/(4+N))){
        return 0;
    } else if (score >= 100*S){
        return N;
    } else {
        return (250*S / (score-50*S)) -4;
    }
}

int main()
{
    cin >> N >> S >> w;
    vector<ll> a(N);
    rep(i, N){
        cin >> a[i];
    }
    
    auto ite = a.begin()+w;
    const ll wp = (*ite)+100*S;
    a.erase(ite);
    N--;
    // show(N);
    sort(a.begin(), a.end(), greater<ll>());
    long double p = 1.0;
    for(const auto& i: a){
        const long double prob = (long double)(calc(wp-i)) / N;
        //        show(prob);
        p *= prob;
        if(prob == 1.0){
            break;
        }
    }
    cout << setprecision(10) << p << endl;
    
    return 0;
}