#include<bits/stdc++.h>
#define int long long
const int inf=8938103643641919514ll;
const int mod=1000000007ll;
const int dd[]={0,-1,0,1,0};
using namespace std;
int a,b,c,d;
struct Node{
	int n=0;
	vector<int> e;
};
Node *ppp;
int p[114514];
signed main(){
	int i,j,k;
	cin>>a>>b>>c;
	for(i=0;i<a;i++)
		cin>>p[i];
	d=p[c]+50*b;
	double ans=1;
	for(i=0;i<a;i++){
		if(p[i]<=p[c])
			continue;
		int x=d-p[i];
		int y=250*b/x-3;
		if(y<1){
			ans=0;
			break;
		}
		ans*=(double)(a-y)/(a-1);
	}
	cout<<setprecision(15)<<ans<<endl;
}