#include <bits/stdc++.h>
using namespace std;
double dp[101];
void solve() {
  int n, m, h, t = 0;
  cin >> n >> m >> h;
  vector<int> s(m);
  for (int& v : s) {
    cin >> v;
    t += v;
  }
  if (t < n) {
    cout << "-1" << '\n';
    return;
  }
  --n, --h, --t;
  --s[h];
  dp[0] = 1;
  for (int i = 0; i < m; ++i) {
    if (i == h) continue;
    while (s[i]--)
      for (int j = n - 1; j >= 0; --j) dp[j + 1] += (n - j) * dp[j] / (t - j);
  }
  cout << fixed << setprecision(10) << 1.0 - dp[n] << '\n';
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
