#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

int main()
{
    int n, s, id;
    cin >> n >> s >> id;
    vector<int> a(n);
    for(int i=0; i<n; ++i)
        cin >> a[i];

    int writer = a[id] + 100 * s;
    a.erase(a.begin() + id);
    -- n;

    vector<int> score(n);
    for(int r=0; r<n; ++r)
        score[n-1-r] = 50 * s + 250 * s / (5 + r);

    sort(a.rbegin(), a.rend());
    double ans = 1.0;
    int k = 0;
    for(int i=0; i<n; ++i){
        while(k < n && a[i] + score[k] <= writer)
            ++ k;
        ans *= max(0, k - i) / (double)(n - i);
    }
    printf("%.10f\n", ans);

    return 0;
}