#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <cstring>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <ctime>
using namespace std;
static const double EPS = 1e-15;
typedef long long ll;
typedef long long LL;
typedef pair<int,int>            PI;
typedef map<PI, int> MPI;
typedef vector<int>     VI;
typedef vector<PI>      VPI;
typedef vector< vector<int> >   VII;
typedef unsigned int UINT32;
typedef unsigned short UINT16;
typedef unsigned char UINT8;
#define ALL(c) (c).begin(), (c).end()
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, s, n) for (int i = (s); i < (int)(n); i++)
#define PB(a)  push_back(a)
#define MP(a,b)  make_pair(a,b)
#define MEMZ(a) memset(a,0,sizeof(a))
#define ITER(c) __typeof((c).begin())
#define PRESENT(c, e) ((c).find((e)) != (c).end())
#define CPRESENT(c, e) (find(all(c), (e)) != (c).end())
#define TR(c, i) for (ITER(c) i = (c).begin(); i != (c).end(); ++i)

#define INF INT_MAX

#define MOD 1000000007

int main(void)
{
  int n,m,h;
  int sum = 0;
  int sum_h = 0;
  cin>>n>>m>>h;
  REP(i,m)
  {
    int a;
    cin>>a;
    sum += a;
    if((i+1)==h)sum_h = a;
  }

  double p = -1.0;
  if(sum>=n)
  {
    sum -= 1;
    sum_h -=1;
    p = 1.0;
    if((sum-sum_h) >= (n-1))
    {
      double d = 1.0;
      for(int i = sum - sum_h + 1; i <= sum ; i++)
      {
        d /= i;
      }
      for(int i = sum - sum_h - (n-1) + 1; i <= sum - (n-1) ; i++)
      {
        d *= i;
      }
      p -= d;
    }
  }

  cout.precision(10);
  cout<<p<<endl;
  return 0;
}