#include <bits/stdc++.h>
int main() {
  int n, m, h, i, cou, t, k;
  double ans;
  while (scanf("%d%d%d", &n, &m, &h) != EOF) {
    for (cou = 0, i = 1; i <= m; ++i) {
      scanf("%d", &k);
      cou += k;
      if (i == h) t = k - 1;
    }
    if (cou < n)
      printf("-1\n");
    else {
      ans = 1;
      for (i = 1; i < n; ++i) ans *= (1 - t * 1.0 / (cou - i));
      printf("%.7lf\n", 1 - ans);
    }
  }
  return 0;
}
