#include <bits/stdc++.h>
using namespace std;
int a[1005];
int main() {
  int n, m, h, i;
  while (scanf("%d%d%d", &n, &m, &h) != EOF) {
    int sum = 0;
    for (i = 0; i < m; i++) {
      scanf("%d", &a[i]);
      sum += a[i];
    }
    if (sum < n) {
      printf("-1\n");
      continue;
    }
    double output;
    if (sum - a[h - 1] < n - 1)
      output = 1;
    else {
      output = 1;
      for (i = 0; i < n - 1; i++)
        output *= sum - a[h - 1] - n + 2 + i, output /= sum - n + 1 + i;
      output = 1 - output;
    }
    printf("%lf\n", output);
  }
  return 0;
}
