#include <fstream>
#include <iomanip>
#include <ctime>
#include <cstdlib>
using namespace std;
ifstream in("cifre.in");
ofstream out("cifre.out");

const int nmax = 1000000;
int a, b, c, k, dif, cnt;
double rasp;

bool Check(int x)
{
    int cnt = 0;
    do {
        cnt += (x%10 == c);
        x /= 10;
    } while (x);

  return cnt>=k;
}

int main() {
    int player_unu=0;

    in>>a>>b>>c>>k;

    dif = b - a + 1;
    if(dif<=nmax)
    {
        for(int i = a; i<=b; i++)
            cnt += Check(i);
        rasp = double(cnt) / dif;
    }
    else
    {
        srand(time(0));
        for(int i = 0; i<nmax; i++)
            cnt += Check(rand() % dif + a);
        rasp = double(cnt) / nmax;
    }

    out<<setprecision(4)<<fixed<<rasp<< "\n";

    return player_unu;
}