#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; }