#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll md = 1000000007;
ll exp(ll a, ll b) {
  ll r = 1ll;
  while (b > 0) {
    if (b & 1) {
      r = r * (a % md);
      r = (r + md) % md;
    }
    b >>= 1;
    a = (a % md) * (a % md);
    a = (a + md) % md;
  }
  return (r + md) % md;
}
ll gcd(ll a, ll b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
ll Min(ll a, ll b) {
  if (a < b) return a;
  return b;
}
ll Max(ll a, ll b) {
  if (a > b) return a;
  return b;
}
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll n, m, h, sum = 0;
  cin >> n >> m >> h;
  --h;
  --n;
  vector<ll> v(m);
  for (ll i = 0; i <= m - 1; ++i) {
    cin >> v[i];
    if (h != i)
      sum += v[i];
    else
      --v[i];
  }
  if (sum < n) {
    if (sum + v[h] < n)
      cout << "-1";
    else
      cout << "1";
    return 0;
  }
  double ans = 1;
  for (ll i = 1; i <= v[h]; ++i) {
    ans = (ans * (sum - n + i) / (sum + i));
  }
  ans = 1 - ans;
  cout << setprecision(9) << fixed << ans;
  cout << '\n';
  return 0;
}
