#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
int verificare(int start, int limit,int cifra,int cate)
{
    int verificate=0;
    for (int i=start;i<=limit;i++)
    {
        int catecf=0;
        int nr2=i;
        while (nr2!=0)
        {
            if (nr2%10==cifra)
            {
                catecf++;
            }
            nr2/=10;
        }
        if (catecf>=cate)
        {
            verificate++;
        }
    }
    return verificate;
}
int main()
{
    int a,b,c,k;
    double total;
    double satisfacute;
    fstream fin("text.in",ios::in);
    fstream fout("text.out",ios::out);
    fin >> a >> b >> c >> k;
    total = b-a+1;
    verificare(a,b,c,k);
    fout << setprecision(4) << verificare(a,b,c,k)/total << '\n';
}