#include <bits/stdc++.h>
using namespace std;
int s[1010];
int main() {
  int n, m, h;
  scanf(" %d %d %d", &n, &m, &h);
  int nstudents = 0;
  for (int i = 1; i <= m; ++i) {
    scanf(" %d", &s[i]);
    nstudents += s[i];
  }
  --n;
  --s[h];
  --nstudents;
  if (n > nstudents) {
    printf("-1.0\n");
    return 0;
  }
  if (n > nstudents - s[h]) {
    printf("1.0\n");
    return 0;
  }
  double ans = 1;
  for (int i = 0; i < n; ++i) {
    ans *= (1 - s[h] / (double)nstudents);
    --nstudents;
  }
  ans = 1 - ans;
  printf("%.10lf\n", ans);
  return 0;
}
