#include <bits/stdc++.h>
using namespace std;
int n, m, k, xl, xr, yl, yr, zl, zr;
double ax, ay, az;
const double EPS = 0.00000001;
vector<int> x, y, z;
int main() {
  int i, j, n, m, h, g = 0;
  cin >> n >> m >> h;
  vector<int> a(m);
  for (i = 0; i < m; i++) {
    cin >> a[i];
    g += a[i];
  }
  if (g < n) {
    puts("-1.0");
    return 0;
  }
  n--;
  g--;
  h--;
  a[h]--;
  if (a[h] == 0) {
    puts("0.0");
    return 0;
  }
  if (g - a[h] < n) {
    puts("1.0");
    return 0;
  }
  double ans = 1;
  for (; n; n--) {
    ans *= (g - a[h] + 0.0) / g;
    g--;
  }
  printf("%.6f", 1 - ans);
  return 0;
}
