#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int A[1010];
int main() {
  int i, j, k;
  int s = 0;
  double r;
  scanf("%d%d%d", &n, &m, &h);
  for (i = 1; i <= m; i++) {
    scanf("%d", A + i);
    s += A[i];
  }
  if (s < n) {
    printf("-1.0\n");
  } else if (s - A[h] < n - 1) {
    printf("1.0\n");
  } else {
    r = 1.0;
    double a, b, c, d;
    a = (double)(s - A[h]);
    b = (double)(s - 1);
    for (i = 0; i < n - 1; i++) {
      r *= (a - i) / (b - i);
    }
    printf("%.9lf\n", 1.0 - r);
  }
  return 0;
}
