#include <bits/stdc++.h>
const int N = 1010;
int n, m, k, h;
int s[N];
int sum;
double s1, s2, r;
void Print();
void PrintData();
int main() {
  scanf("%d %d %d", &n, &m, &h);
  h--;
  for (int i = 0; i < m; i++) {
    scanf("%d", &s[i]);
    sum += s[i];
  }
  if (n > sum)
    printf("-1.0");
  else if (n > sum - s[h] + 1)
    printf("1.0");
  else {
    s1 = sum - s[h], s2 = sum - 1;
    r = 1.0;
    for (int i = 1; i < n; i++) r = r * s1 / s2, s1--, s2--;
    r = 1.0 - r;
    printf("%0.6lf", r);
  }
  return 0;
}
void PrintData() {}
void Print() { printf("\n"); }
