#include <bits/stdc++.h>
using namespace std;
int n, h;
double m, t;
int main() {
  while (scanf("%lf%d%d", &m, &n, &h) == 3) {
    double e = 0;
    double I = 0;
    double r = 1.0;
    for (int i = 1; i <= n; i++) {
      scanf("%lf", &t);
      if (i == h) I = t;
      e += t;
    }
    e = e - 1;
    m--;
    I = I - 1;
    if (e < m * 1.0) {
      puts("-1.0");
      continue;
    }
    if (e - I < m * 1.0) {
      puts("1.0");
      continue;
    }
    for (int i = 0; i < m; i++) {
      r = r * (e - I - i) / (e - i);
    }
    printf("%.10lf\n", 1 - r);
  }
  return 0;
}
