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

#define _p(...) (void)printf(__VA_ARGS__)
#define forr(x,arr) for(auto&& x:arr)
#define _overload3(_1,_2,_3,name,...) name
#define _rep2(i,n) _rep3(i,0,n)
#define _rep3(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,_rep3,_rep2,)(__VA_ARGS__)
#define _rrep2(i,n) _rrep3(i,0,n)
#define _rrep3(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define rrep(...) _overload3(__VA_ARGS__,_rrep3,_rrep2,)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define bit(n) (1LL<<(n))
#define sz(x) ((int)(x).size())
#define fst first
#define snd second
using ll=long long;using pii=pair<int,int>;
using vb=vector<bool>;using vs=vector<string>;
using vi=vector<int>;using vvi=vector<vi>;using vvvi=vector<vvi>;
using vl=vector<ll>;using vvl=vector<vl>;using vvvl=vector<vvl>;
using vd=vector<double>;using vvd=vector<vd>;using vvvd=vector<vvd>;
using vpii=vector<pii>;using vvpii=vector<vpii>;using vvvpii=vector<vvpii>;
template<typename T>T read(){T t;cin>>t;return t;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){o<<'('<<p.fst<<", "<<p.snd<<')';return o;}

double solve() {
  int n = read<int>();
  int star = read<int>();
  int w_id = read<int>();

  int w = 0;
  vector<int> A;

  rep(i, n) {
    if (i == w_id) {
      w = read<int>();
    }
    else {
      A.push_back(read<int>());
    }
  }

  n -= 1;

  vi S(n);
  rep(i, n) {
    S[i] = 50 * star + (500 * star) / (8 + 2 * (i + 1));
  }
  w += S[0];

  reverse(all(S));
  //cout << "S:"; rep(ii,sz(S)) cout << ' ' << S[ii]; cout << endl;

  forr(a, A) {
    a -= w;
    if (a > 0) return 0;
    a = -a;
  }
  sort(all(A));

  double ans = 1;

  rep(i, n) {
    int a = A[i];
    //cout << "a: "<<a << endl;
    int r = upper_bound(all(S), a) - S.begin();
    //cout << "r: "<<r << endl;
    ans *= 1 - (1.0 * (n - r) / (n-i));
  }

  return ans;
}
void Main() {
  _p("%.15f\n", solve());
}
int main() { cin.tie(nullptr); ios::sync_with_stdio(false); Main(); return 0; }