#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:64000000")
using namespace std;
const int INF = (int)1E9;
const long long INF64 = (long long)1E18;
const long double EPS = 1E-8;
const long double PI = 3.1415926535897932384626433832795;
const int MAXN = 1010;
int n, m, a[MAXN], h;
int main() {
  cin >> n >> m >> h;
  for (int i = 0; i < (int)(m); i++) cin >> a[i];
  h--;
  n--;
  a[h]--;
  int sum = 0;
  for (int i = 0; i < (int)(m); i++) sum += a[i];
  if (sum < n) {
    cout << -1 << endl;
    return 0;
  }
  if (sum - a[h] < n) {
    cout << 1 << endl;
    return 0;
  }
  long double ans = 1;
  for (int i = 1; i <= n; i++)
    ans *= (0.0 + sum - a[h] - n + i) / (0.0 + sum - n + i);
  cout.precision(10);
  cout << fixed << 1 - ans << endl;
  return 0;
}
