#include <bits/stdc++.h>
using namespace std;
int s[1005], S;
int main() {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  for (int i = 1; i <= m; i++) {
    scanf("%d", &s[i]);
    S += s[i];
  }
  if (n > S)
    printf("%d\n", -1);
  else {
    S--;
    s[h]--;
    double p = 1;
    for (int i = 1; i < n; i++) p *= (S - s[h]) * 1.0 / S, S--;
    printf("%lf\n", 1 - p);
    return 0;
  }
}
