#include <bits/stdc++.h>
using namespace std;
int num, dep, h;
int sum, cc;
double nCr(int n, int r) {
  if (r < n - r) r = n - r;
  double res = 1.0;
  for (int i = r + 1; i <= n; i++) {
    res *= (double)i;
    res /= (double)(i - r);
  }
  return res;
}
int main(void) {
  scanf("%d%d%d", &num, &dep, &h);
  for (int i = 1; i <= dep; i++) {
    int x;
    scanf("%d", &x);
    sum += x;
    if (i == h) cc = x - 1;
  }
  sum -= 1;
  num -= 1;
  if (sum < num) {
    printf("-1\n");
    return 0;
  }
  double nue = 0.0, den = nCr(sum, num);
  for (int i = 1; i <= min(cc, num); i++) {
    if (num - i <= sum - cc) nue += (nCr(cc, i) * nCr(sum - cc, num - i));
  }
  printf("%.8f\n", nue / den);
  return 0;
}
