#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000 + 5;
int n, m, h, a[maxn];
int main() {
  scanf("%d%d%d", &n, &m, &h);
  int sum = 0;
  for (int i = 1; i <= m; i++) scanf("%d", &a[i]), sum += a[i];
  if (sum < n) return 0 * puts("-1");
  n--;
  sum--;
  a[h]--;
  if (sum - a[h] < n) return 0 * puts("1");
  double ans = 1.0;
  double x = sum - a[h], y = sum;
  for (int i = 1; i <= n; i++) {
    ans *= x / y;
    x--;
    y--;
  }
  printf("%.10f\n", 1.0 - ans);
  return 0;
}
