#include <bits/stdc++.h>
using namespace std;
const int N = 1e3 + 5;
int S[N];
int main() {
  int n, m, h;
  scanf("%d %d %d", &n, &m, &h);
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &S[i]);
    sum += S[i];
  }
  if (sum < n) {
    printf("-1\n");
    return 0;
  }
  sum -= S[h];
  n--;
  double ans = 1;
  for (int i = 1; i < S[h]; i++) {
    ans *= (double)(sum - n + i) / (sum + i);
  }
  ans = 1 - ans;
  printf("%.12lf\n", ans);
  return 0;
}
