#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int s[1010];
int sum;
int main() {
  while (cin >> n >> m >> h) {
    sum = 0;
    for (int i = 0; i < m; ++i) {
      cin >> s[i];
      sum += s[i];
    }
    if (sum < n) {
      puts("-1.0");
      continue;
    }
    double res = 1.0;
    int H = s[h - 1];
    if (sum - H >= n - 1) {
      res = 1.0;
      for (int i = 1; i < H && sum - H >= n - 1; ++i) {
        res = res * (double)(sum - n + 1 - i) / (double)(sum - i);
      }
    } else {
      res = 0.0;
    }
    printf("%.7lf\n", 1.0 - res);
  }
  return 0;
}
