#include <bits/stdc++.h>
using namespace std;
inline long long bip(long long x, long long n, long long mod) {
  long long res = 1;
  while (n) {
    if (n & 1) {
      res = (res * x) % mod;
    }
    x = (x * x) % mod;
    n >>= 1;
  }
  return res;
}
const int maxn = 1e9;
const long long inf = 1e18;
const int mod = 1e9 + 7;
const int N = 1e3 + 10;
int n, m, a[1010], ind;
long long C[10][10];
void Cnk() {
  for (int i = 1; i < 10; i++) {
    C[i][0] = C[i][i] = 1;
    for (int j = 1; j < i; j++) {
      C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
    }
  }
}
void solve() {
  cin >> n >> m >> ind;
  for (int i = 1; i <= m; i++) cin >> a[i];
  n--, a[ind]--;
  int sum = 0;
  for (int i = 1; i <= m; i++) sum += a[i];
  if (sum < n) {
    cout << -1;
    return;
  }
  if (a[ind] == 0) {
    cout << 0.00000;
    return;
  }
  int k = sum - a[ind], l = sum, x = l - n, y = k - n;
  if (y < 0) {
    cout << 1.00000000;
    return;
  }
  long double up = 1, down = 1;
  for (int i = k - n + 1; i <= l - n; i++) up *= (i + 0.0);
  for (int i = k + 1; i <= l; i++) down *= (i + 0.0);
  cout << 1.0000000000 - up / down;
}
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int tt = 1;
  cout << fixed << setprecision(9);
  while (tt--) {
    solve();
    cout << '\n';
  }
}
