#include <bits/stdc++.h>
using namespace std;

int score(int star,int rank){
	return (int)(50.0L*(long double)star+50.0L*(long double)star/(0.8L+0.2L*rank));
}

void test(){
	int star = 3;
	for(int rank=1;rank<=10;rank++){
		cout<<rank<<" "<<score(star,rank)<<endl;
	}
}

int N,star,writer;//,tester;

int main(){
//	test();
	vector<int> scores;
	int score_writer;//, score_tester;
	cin>>N>>star>>writer;//>>tester;
	for(int i=0;i<N;i++){
		int s;
		cin>>s;
		if(i==writer)score_writer = s + star*100;
//		else if(i==tester)score_tester = s + star*100;
		else scores.push_back(s);
	}
	sort(scores.begin(),scores.end());
	/*
	if(score_tester>score_writer){
		printf("0.0\n");
		return 0;
	}
	*/
	long double ans = 1.0L;
	int j = 0;
	for(int rank=1;rank<=N-1;rank++){
		int s = score(star,rank);
		while(j<scores.size() and scores[j]+s<=score_writer)j++;
//		cerr<<" "<<j-(rank-1)<<" "<<N-rank<<endl;
		ans*=(j-(rank-1))/(long double)(N-rank);
	}
	printf("%.12Lf\n",ans);
	return 0;
}