#include <bits/stdc++.h>
using namespace std;
const long long MAXN = 1003;
long long n, m, h;
long double a[MAXN];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m >> h;
  for (long long i = 1; i <= m; ++i) cin >> a[i];
  swap(a[h], a[1]);
  a[1] -= 1.0l, n -= 1.0l;
  long double allp = accumulate(a + 1, a + m + 1, 0.0l), notp = allp - a[1];
  if (n > allp) {
    cout << "-1\n";
    return 0;
  }
  long double res = 1;
  for (long long cnt = 0; cnt < n; ++cnt, notp -= 1.0l, allp -= 1.0l) {
    res = (res * notp) / allp;
  }
  cout << fixed << setprecision(6) << 1.0l - res;
}
