#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1010;
const int MAXQ = 100010;
const double EPS = 1e-8;
const double DINF = 1e30;
const int IINF = ~0U >> 3;
const long long LINF = 1LL << 61;
void debug() { cout << "error_here" << endl; }
int h, n, m;
int sum, p;
double s1, s2;
void init() {
  scanf("%d%d%d", &n, &m, &h);
  sum = 0;
  for (int i = 1, a; i <= m; ++i) {
    scanf("%d", &a);
    sum += a;
    if (i == h) p = a;
  }
  if (sum < n) {
    puts("-1.0");
    return;
  }
  double ans = 1;
  for (int i = sum - p - n + 2; i <= sum - p; ++i)
    ans *= (double)i / (i + p - 1);
  printf("%.6f\n", 1 - ans);
}
int main() {
  init();
  return 0;
}
