#include <bits/stdc++.h>
using namespace std;
const int N = 1005;
int n, m, h, a[N], tot;
double s = 1;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m >> h;
  for (int i = 1; i <= m; ++i) cin >> a[i], tot += a[i];
  if (tot < n) return cout << -1, 0;
  for (int i = 1; i < n; ++i) {
    s = s * 1.0f * (double)((double)tot + 1 - a[h] - i) / (double)(tot - i);
  }
  cout << setprecision(6) << fixed << 1.0 - s;
}
