#include <bits/stdc++.h>
using namespace std;
struct debugger {
  template <typename T>
  debugger& operator,(const T& v) {
    cerr << v << "\t";
    return *this;
  }
} dbg;
template <class T>
void setmax(T& a, T b) {
  if (a < b) a = b;
}
template <class T>
void setmin(T& a, T b) {
  if (b < a) a = b;
}
template <class T>
T Abs(T x) {
  return x > 0 ? x : -x;
}
template <class T>
inline T sqr(T x) {
  return x * x;
}
template <class T>
inline T Mod(T n, T m) {
  return (n % m + m) % m;
}
template <class T>
string toString(T n) {
  ostringstream oss;
  oss << n;
  oss.flush();
  return oss.str();
}
long long toInt(string s) {
  long long r = 0;
  istringstream sin(s);
  sin >> r;
  return r;
}
bool isVowel(char ch) {
  ch = tolower(ch);
  if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u')
    return true;
  return false;
}
bool isUpper(char c) { return c >= 'A' && c <= 'Z'; }
bool isLower(char c) { return c >= 'a' && c <= 'z'; }
long long Pow(long long B, long long P) {
  long long R = 1;
  while (P > 0) {
    if (P % 2 == 1) R = (R * B);
    P /= 2;
    B = (B * B);
  }
  return R;
}
long long BigMod(long long B, long long P, long long M) {
  long long R = 1;
  while (P > 0) {
    if (P % 2 == 1) {
      R = (R * B) % M;
    }
    P /= 2;
    B = (B * B) % M;
  }
  return R;
}
void binprint(long long mask, long long n) {
  long long i;
  string s = "";
  do {
    s += (mask % 2 + '0');
    mask /= 2;
  } while (mask);
  reverse(s.begin(), s.end());
  s = string(max(n - (int)s.size(), 0LL), '0') + s;
  for (i = (int)s.size() - n; i < (int)s.size(); i++) printf("%c", s[i]);
  printf("\n");
}
void ASCII_Chart() {
  long long i, j, k;
  printf("ASCII Chart:(30-129)\n");
  for (i = 30; i < 50; i++) {
    for (j = 0; j < 5; j++) {
      k = i + j * 20;
      printf("%3d---> '%c'   ", k, k);
    }
    printf("\n");
  }
}
int s[1001];
long double bc[100010][105];
double cal(int n, int m, int h, int sum) {
  h--;
  int i, j;
  double ans = 0;
  if (sum < n) return -1.0;
  sum--;
  n--;
  s[h]--;
  if (!s[h] || !n) return 0;
  if (sum - n < s[h]) return 1;
  memset(bc, 0, sizeof(bc));
  for (i = 0; i <= sum; i++) {
    bc[i][0] = 1;
    for (j = 1; j <= min(i, 100); j++)
      bc[i][j] = bc[i - 1][j - 1] + bc[i - 1][j];
  }
  return 1 - bc[sum - s[h]][n] / bc[sum][n];
}
int main() {
  int i, j, test, Case = 1, sum = 0, n, h, m;
  while (scanf("%d%d%d", &n, &m, &h) == 3) {
    sum = 0;
    for (i = 0; i < m; i++) {
      scanf("%d", &s[i]);
      sum += s[i];
    }
    double ans = cal(n, m, h, sum);
    printf("%.10lf\n", ans);
  }
  return 0;
}
