#include <cstdio>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
#include <cmath>
#include <iostream>
#include <ctime>
#include <utility>
#include <string>
#include <memory.h>
using namespace std;

#define forn( i,n ) for ( int i=0; i<(int)(n); i++ )
#define sz(a) (int)((a).size())
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef pair<int,int> pii;

ld getf( int mask )
{
  if ( memf[mask] > -0.5 ) return memf[mask];

  ld& res = memf[mask];
  res = 0;

  int fs = -1;
  forn( i, n )
    if ( mask & ( 1<<i ) )
    {
      if ( fs == -1 ) fs = i;
      else
      {
        if ( bc[mask] == start-k && i == need ) return res = ( i-lp )*1.0 / n;
        res += getf(
      }
    }
}

int main()
{
  freopen( "c.in", "r", stdin );
  freopen( "c.out", "w", stdout );

  scanf( "%d %d %d", &n, &need, &k );
  k--; need--;
  forn( i, n ) scanf( "%d", &x ), mask |= ( 1<<x );

  bc[0] = 1;
  for ( int i=1; i<(1<<20); i++ ) bc[i] = bc[i&(i-1)] + 1;

  startbc = bc[mask];
  if ( k > startbc ) printf( "0\n" );
  else printf( "%.7f\n", getf( mask ) );

  return 0;
}