#include <bits/stdc++.h>
using namespace std;
const int SIZE = 1000 + 5;
int li[SIZE];
int main(void) {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  for (register int i = 1; i <= m; ++i) scanf("%d", &li[i]);
  const int sum = accumulate(li + 1, li + m + 1, 0) - 1;
  --li[h], --n;
  if (sum < n) {
    printf("-1\n");
    return 0;
  }
  if (sum - li[h] < n) {
    printf("1.0\n");
    return 0;
  }
  double ans = 1.0;
  for (register int i = n + 1; i <= sum - li[h]; ++i) ans = ans * i / (i - n);
  for (register int i = n + 1; i <= sum; ++i) ans = ans * (i - n) / i;
  printf("%.8f\n", 1.0 - ans);
  return 0;
}
