#include <bits/stdc++.h>
using namespace std;
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << endl;
  err(++it, args...);
}
long long int power(long long int a, long long int b,
                    long long int m = 1000000007) {
  long long int ans = 1;
  a = a % m;
  while (b > 0) {
    if (b & 1) ans = (1LL * a * ans) % m;
    b >>= 1;
    a = (1LL * a * a) % m;
  }
  return ans;
}
long long int ncr(long long int n, long long int r) {
  long long int res = 1;
  if (r > n - r) r = n - r;
  for (long long int i = 0; i < r; i++) {
    res *= n - i;
    res /= i + 1;
  }
  return res;
}
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long int lcm(long long int a, long long int b) {
  return (a * b) / gcd(a, b);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int tt = 1;
  long long int ii = 0;
  while (tt-- && ++ii) {
    long long int n, m, h;
    cin >> n >> m >> h;
    h--;
    vector<long long int> a(m);
    long long int sum = 0;
    for (long long int i = 0; i < m; i++) cin >> a[i], sum += a[i];
    if (sum < n) return cout << -1, 0;
    if (a[h] == 1) return cout << 0, 0;
    long double ans = 1.0;
    for (long long int i = 0; i < n - 1; i++) {
      ans *= (long double)(sum - a[h] - i) / (sum - 1 - i);
    }
    cout << 1.0 - ans;
  }
  return 0;
}
