// Merry Christmas! (1発で通ってくれ)

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))

// mod
const ll MOD = 1000000007ll;
#define FIX(a) ((a)%MOD+MOD)%MOD

// floating
typedef double Real;
const Real EPS = 1e-11;
#define EQ0(x) (abs(x)<EPS)
#define EQ(a,b) (abs(a-b)<EPS)
typedef complex<Real> P;

int n,s,id;
int a[83025];

int main(){
  scanf("%d%d%d",&n,&s,&id);
  REP(i,n)scanf("%d",a+i);

  int writerscore = s*100 + a[id];
  a[id] = 1e9;
  sort(a,a+n);

  double ans = 1.0;
  int ok_line = 0;
  REP(i,n-1){
    int score = (450*s + 50*s*(i+1)) / (5+i);
    while(ok_line<n-1){
      if(score + a[ok_line] <= writerscore)ok_line++;
      else break;
    }
    int choice = ok_line - i;
    int all = n-1-i;
    if(choice <= 0){
      ans = 0.0;
      break;
    }else{
      ans *= (double)choice / all;
    }
  }
  printf("%.12f\n",ans);
  return 0;
}