#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(char c) { return to_string(string(1, c)); }
string to_string(vector<bool> v) {
  string res = "{";
  int sz = (int)v.size();
  for (int i = 0; i < sz; i++) {
    if (i > 0) res += ", ";
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& a : v) {
    if (!first) res += ", ";
    first = false;
    res += to_string(a);
  }
  res += "}";
  return res;
}
void debug_out() { cerr << endl; }
template <typename A, typename... B>
void debug_out(A a, B... b) {
  cerr << to_string(a);
  if (sizeof...(b)) cerr << ", ";
  debug_out(b...);
}
int main() {
  cin.tie(0);
  cin.exceptions(cin.failbit);
  ios_base::sync_with_stdio(0);
  int n, m, h;
  cin >> n >> m >> h;
  vector<int> a(m + 1);
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    cin >> a[i];
    sum += a[i];
  }
  if (n > sum) {
    cout << -1 << '\n';
    return 0;
  }
  ld first = 1;
  for (int i = 1; i < a[h]; i++) {
    first *= sum - i;
  }
  ld second = 1;
  for (int i = 1; i < a[h]; i++) {
    second *= (sum - i) - (n - 1);
  }
  42;
  ld ans = second / first;
  cout << setprecision(10) << fixed << 1 - ans << '\n';
}
