#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  h--;
  n--;
  int cnto = 0, cntr = 0, tot;
  for (int i = 0; i < m; i++) {
    int t;
    scanf("%d", &t);
    if (i == h)
      cntr = t - 1;
    else
      cnto += t;
  }
  if (n > cnto + cntr) {
    printf("-1.0\n");
    return 0;
  }
  tot = cnto + cntr;
  double prob = 1;
  for (int i = n; i > 0; i--, cnto--, tot--) {
    prob *= (double)cnto / tot;
  }
  printf("%.8lf\n", 1 - prob);
  return 0;
}
