#include <bits/stdc++.h>
using namespace std;
int s[110];
int main() {
  int n, m, h, sx = 0;
  scanf("%d%d%d", &n, &m, &h);
  for (int i = 0; i < m; ++i) {
    scanf("%d", s + i);
    sx += s[i];
  }
  if (sx < n) {
    printf("-1.0000000000\n");
    return 0;
  }
  if (s[h - 1] < 2) {
    printf("0.0000000000\n");
    return 0;
  }
  if (sx == n) {
    printf("1.0000000000\n");
    return 0;
  }
  int ss = sx - 1;
  --n, --s[h - 1];
  int sn = ss - s[h - 1];
  double ans = 1.0;
  for (int i = sn - n + 1; i <= ss - n; ++i) {
    ans = ans * i;
  }
  for (int i = sn + 1; i <= ss; ++i) {
    ans = ans / i;
  }
  printf("%.10f\n", 1 - ans);
  return 0;
}
