#include <bits/stdc++.h>
using namespace std;
const int N = 1000010;
int n, m, h, a, b, s;
int main() {
  scanf("%d %d %d", &n, &m, &h);
  for (int i = 1; i <= m; i++) {
    int x;
    scanf("%d", &x);
    if (i == h)
      a = x - 1;
    else
      b += x;
    s += x;
  }
  if (s < n) {
    printf("-1\n");
    return 0;
  }
  if (s == n) {
    if (a)
      printf("1\n");
    else
      printf("0\n");
    return 0;
  }
  double z = 1;
  for (int i = 1; i <= n - 1; i++) {
    double k = b - i + 1 + 0.0;
    double l = a + b - i + 1 + 0.0;
    z *= k / l;
  }
  printf("%.10lf\n", 1.0 - z);
  return 0;
}
