#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

int N, S, wi;
ll A[100000];
ll LIM = -1;

ll f(int r) {
    return 50ll * S + 500ll * S / (8 + 2 * r);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    cin >> N >> S >> wi;
    rep(i, N)cin >> A[i];
    swap(A[wi], A[0]);

    LIM = A[0] + 100ll * S;

    // 得点降順
    deque<ll> pt;
    for(int r = 1; r <= N - 1;++r) {
        pt.push_back(f(r));
    }

    sort(A + 1, A + N, greater<ll>());

    double p = 1;
    for(int i = 1; i < N; ++i) {
        // a以下のポイントはOK
        ll a = LIM - A[i];
        auto it = lower_bound(all(pt), a, greater<ll>());
        if(it == pt.end()) {
            p = 0;
            break;
        }
        p *= (double)distance(it, pt.end()) / sz(pt);
        pt.pop_back();
    }

    cout << setprecision(20) << p << endl;
}