#include <bits/stdc++.h>
using namespace std;
int T, m, n, q, w, mi, ma, cnt, s, t, fi, cc, e, r, z, x, h;
double an;
int a[1010];
int main() {
  scanf("%d%d%d", &n, &m, &h);
  h--;
  s = 0;
  for (int i = 0; i < m; i++) scanf("%d", &a[i]), s += a[i];
  if (s < n) {
    printf("%.10f\n", -1.0);
    return 0;
  }
  w = s - a[h];
  s--;
  an = 1.0;
  for (int i = 0; i < n - 1; i++) {
    an *= ((double)w / (double)s);
    w--;
    s--;
  }
  printf("%.10f\n", 1.0 - an);
  return 0;
}
