#include <bits/stdc++.h>
using namespace std;
template <class T1>
void deb(T1 e1) {
  cout << e1 << endl;
}
template <class T1, class T2>
void deb(T1 e1, T2 e2) {
  cout << e1 << " " << e2 << endl;
}
template <class T1, class T2, class T3>
void deb(T1 e1, T2 e2, T3 e3) {
  cout << e1 << " " << e2 << " " << e3 << endl;
}
template <class T1, class T2, class T3, class T4>
void deb(T1 e1, T2 e2, T3 e3, T4 e4) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << endl;
}
template <class T1, class T2, class T3, class T4, class T5>
void deb(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << " " << e5 << endl;
}
template <class T1, class T2, class T3, class T4, class T5, class T6>
void deb(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5, T6 e6) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << " " << e5 << " " << e6
       << endl;
}
double nCr(int n, int r) {
  if (r == 0) return 1;
  if (r > n) return 0;
  r = min(r, n - r);
  double ret = 1.0;
  for (int i = 1; i <= r; i++) {
    ret *= (double)(n - r + i) / (double)i;
  }
  return ret;
}
int arr[100000 + 10];
int main() {
  int t, kase = 0, i, j, k, l;
  int n, m, h, sum;
  while (scanf("%d %d", &n, &m) == 2) {
    scanf("%d", &h);
    sum = 0;
    for (i = 1; i <= m; i++) {
      scanf("%d", &arr[i]);
      sum += arr[i];
    }
    arr[h]--;
    n--;
    sum--;
    if (sum < n)
      printf("-1\n");
    else {
      double ans = 0;
      for (i = 1; i <= min(arr[h], n); i++) {
        ans += nCr(arr[h], i) * nCr(sum - arr[h], n - i);
      }
      ans = ans / nCr(sum, n);
      ans = min(ans, 1.0);
      printf("%lf\n", ans);
    }
  }
  return 0;
}
