#include <bits/stdc++.h>
using namespace std;
long double fac(int x) {
  long double ans = 1;
  for (int i = 2; i <= x; i++) {
    ans *= i;
  }
  return ans;
}
long double ncr(int n, int r) {
  if (n < r) {
    return 0;
  }
  r = min(r, n - r);
  long double num = 1, den = fac(r);
  for (int i = n; i >= n - r + 1; i--) {
    num *= i;
  }
  return num / den;
}
int main() {
  int n, m, h;
  scanf("%d %d %d", &n, &m, &h);
  h--;
  vector<int> s(m);
  int tot = -1;
  int x, y;
  for (int i = 0; i < m; i++) {
    scanf("%d", &s[i]);
    tot += s[i];
    if (i == h) {
      x = s[i] - 1;
    }
  }
  y = tot - x;
  n--;
  if (x + y < n) {
    printf("-1\n");
  } else {
    printf("%.10Lf\n", 1 - ncr(y, n) / ncr(x + y, n));
  }
}
