//#pragma GCC target("avx2")
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using pli = pair<ll,int>;
#define TEST cerr << "TEST" << endl
#define AMARI 998244353
//#define AMARI 1000000007
#define el '\n'
#define El '\n'
#define eps 1e-6

//r位を取った時スコアはいくつになるか
ll func(ll score,int r,ll s){
    ll ans = 500 * s / (8 + 2 * r);
    ans += 50 * s;
    ans += score;
    return ans;
}

#define MULTI_TEST_CASE false 
void solve(void){
    int n;
    ll s;
    int id;
    cin >> n >> s >> id;
    vector<ll> a(n);
    for(int i = 0; i < n; i++)cin >> a[i];
    swap(a[id],a[n - 1]);
    ll writer_score = a.back() + 100LL * s;
    a.pop_back();
    n--;
    sort(a.rbegin(),a.rend());
    long double ans = 1;
    cout << fixed << setprecision(15);
    int bunbo = n;
    for(int i = 0; i < n; i++){
        //もし(N-i)位をとっても writer_score をオーバーするなら0を出力
        if(func(a[i],n - i,s) > writer_score){
            cout << 0 << el;
            return;
        }
        //cerr << func(a[i],1,s) << el;
        //もし1位をとっても writer_score をオーバーしないならそのまま
        if(func(a[i],1,s) <= writer_score)continue;
        
        //何位以下じゃないといけないかにぶたん
        int l = 0,r = n - i + 1;
        while(r - l >= 2){
            int c = (r + l) / 2;
            if(func(a[i],c,s) <= writer_score)r = c;
            else l = c;
        }
        
        //cerr << l << ' ' << r << el;
        for(int j = r; j >= l - 1; j--){
            if(func(a[i],j,s) > writer_score){
                //j+1位がギリギリOKで、j位はダメ
                //一番下の取りうる順位はn
                int bunsi = 0,bunbo = 0;
                bunbo = n - i;
                bunsi = bunbo - j;
                
                //cerr << bunsi << ' ' << bunbo << el;
                ans *= (long double)bunsi / (long double)bunbo;
                break;
            }
        }
        
    }
    cout << ans << El;
    return;
}

void calc(void){
    return;
}

signed main(void){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    calc();
    int t = 1;
    if(MULTI_TEST_CASE)cin >> t;
    while(t--){
        solve();
    }
    return 0;
}