#include <bits/stdc++.h>
using namespace std;
int sumoth = 0, myteam;
int main() {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  n--;
  for (int i = 1; i <= m; i++) {
    int a;
    scanf("%d", &a);
    if (i == h)
      myteam = a - 1;
    else
      sumoth += a;
  }
  if (n > myteam + sumoth)
    printf("-1.0\n");
  else {
    double ans = 1;
    for (int i = 0; i < n; i++) {
      ans *= (sumoth - i);
      ans /= (sumoth + myteam - i);
    }
    printf("%.7lf\n", 1 - ans);
  }
  return 0;
}
