#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int a[1005];
double f() {
  int s1 = 0;
  int s2 = 0;
  for (int i = 0; i < m - 1; ++i) s1 += a[i];
  for (int i = 0; i < m; ++i) s2 += a[i];
  if (s2 < n) return -1.0;
  if (s1 < n) return 1.0;
  double res = 1.0;
  for (int i = 0; i < n; ++i) {
    res *= s1;
    res /= s2;
    --s1;
    --s2;
  }
  return 1.0 - res;
}
int main() {
  cin >> n >> m >> h;
  for (int i = 0; i < m; ++i) cin >> a[i];
  --h;
  --n;
  --a[h];
  swap(a[h], a[m - 1]);
  printf("%.10lf\n", f());
  return 0;
}
