#include <bits/stdc++.h>
using namespace std;
int i, j;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  int sum1 = 0, sum2 = 0;
  for (i = 1; i <= m; i++) {
    int x;
    cin >> x;
    sum1 += x;
    if (i - h) {
      sum2 += x;
    }
  }
  n--;
  sum1--;
  if (sum1 < n) {
    cout << -1 << endl;
    return 0;
  }
  if (sum1 == sum2) {
    cout << 0 << endl;
    return 0;
  }
  vector<int> a, b;
  for (i = sum2 - n + 1; i <= sum2; i++) {
    a.push_back(i);
  }
  for (i = sum1 - n + 1; i <= sum1; i++) {
    b.push_back(i);
  }
  long double ans = 1;
  for (i = 0; i < n; i++) {
    ans *= a[i];
    ans /= b[i];
  }
  cout << 1 - ans << endl;
}
