#include <bits/stdc++.h>
using namespace std;
const int M = 1111;
int n, m, h, s[M];
int main() {
  scanf("%d%d%d", &n, &m, &h);
  for (int i = 1; i <= m; ++i) {
    scanf("%d", s + i);
  }
  swap(s[h], s[m]);
  n -= 1;
  int sum[2] = {0, 0};
  for (int i = 1; i < m; ++i) {
    sum[0] += s[i];
  }
  sum[1] = sum[0] + s[m] - 1;
  if (sum[1] < n) {
    printf("-1.0\n");
  } else {
    double result = 1.0;
    for (int i = 0; i < n; ++i) {
      result = result * (sum[0] - i) / (sum[1] - i);
    }
    printf("%.8f\n", 1 - result);
  }
  return 0;
}
