#include <iostream>
#include <vector>
#include <time.h>
#include <stdlib.h>
using namespace std;

const size_t NTESTS = 10000;
vector<char> deck;

void FY_shuffle (vector<char> & v)
{
    for (size_t i=1;i<v.size();++i)
        swap(v[i],v[rand()%(i+1)]);
}

int main ()
{
    deck.resize(52);
    for (size_t i=0;i<26;++i)
        deck[i]='R'; // R - красная
    for (size_t i=26;i<52;++i)
        deck[i]='B'; // B - чёрная
    srand(time(NULL));
    FY_shuffle(deck);
    double average=0;
    for (size_t j=0;j<NTESTS;++j)
    {
        int correct_answers=0,reds=26,blacks=26;
        for (size_t i=0;i<deck.size();++i)
        {
            int r=rand()%(reds+blacks);
            if (r<reds)
            {
                if (deck[i]=='R')
                    --reds,++correct_answers;
                else
                    --blacks;
            }
            else
            {
                if (deck[i]=='B')
                    --blacks,++correct_answers;
                else
                    --reds;
            }
        }
        average+=(double)correct_answers/(double)NTESTS;
    }
    cout<<average<<endl;
    return 0;
}