#include <bits/stdc++.h>
using namespace std;
int n, m, h, s[100100];
int tot;
int main() {
  scanf("%d %d %d", &n, &m, &h);
  for (int i = 0; i < m; ++i) {
    scanf("%d", &s[i]);
    tot += s[i];
  }
  if (tot < n)
    printf("-1.0\n");
  else {
    double rs = 1.0;
    for (int i = 0; i < n - 1; ++i) {
      rs = rs * (tot - s[h - 1] - i) / double(tot - i - 1);
    }
    printf("%.12lf\n", 1 - rs);
  }
  return 0;
}
