#include <bits/stdc++.h>
using namespace std;
int num[105];
double cal(int w, int e, int s) {
  if (e < s) return 1.0;
  double res = 1, dw = (double)w, de = (double)e;
  for (int i = 0; i < s; i++) {
    res = res * de / dw;
    dw--;
    de--;
  }
  return 1.0 - res;
}
int main() {
  int n, m, h;
  int s;
  while (~scanf("%d%d%d", &n, &m, &h)) {
    s = 0;
    for (int i = 1; i <= m; i++) {
      scanf("%d", &num[i]);
      s += num[i];
    }
    if (s < n)
      printf("-1.0\n");
    else {
      printf("%.7lf\n", cal(s - 1, s - num[h], n - 1));
    }
  }
  return 0;
}
