#include <bits/stdc++.h>
using namespace std;
int s[1005];
int n, m, h;
int main() {
  scanf("%d%d%d", &n, &m, &h);
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &s[i]);
    sum += s[i];
  }
  if (sum < n) {
    cout << "-1";
    return 0;
  }
  if (sum - s[h] < n - 1) {
    cout << "1.0";
    return 0;
  }
  double a = sum - n - s[h] + 1, b = sum - s[h];
  double rs = 1.0;
  for (int i = 1; i < s[h]; i++) rs = rs * ((a + i) / (b + i));
  printf("%.9f", 1 - rs);
}
