#include <fstream>
#include <iostream>
using namespace std;
long long till(long long a,long long c,long long k)
{
    if ((a==0)&&(c==0)&&(k==1))
    return 1;
    long long pow10[10];
    pow10[0]=1;
    for (long long i=1; i<=10; i++)
        pow10[i]=pow10[i-1]*10;

    long long total=0,nrDig=0;

    for (long long i=0; i<1<<nrDig; i++)
    {
        //pentru fiecare posibilitate
        long long nr1=0;
        for (long long j=0; j<nrDig; j++)
            if (i&(1<<j))
                nr1++;
        //nr de 1-uri
        long long prefix=0;
        bool act=false;
        nrDig=10;
        if (nr1 >= k)
        {
            for (long long j=nrDig-1; j>=0; j--)
            {
                if (i&(1<<j))
                {
                    if ((prefix<=0)&&(c==0))
                    {
                        prefix=-1;
                        break;
                    }
                    if (!act)
                    {
                        if ((a/pow10[j])%10>c)
                        {
                            act = true;
                        }

                        else if ((a/pow10[j])%10<c)
                        {
                            if (prefix==0)
                            {
                                prefix=-1;
                                break;
                            }
                            prefix--;
                            act = true;
                        }

                    }

                }
                else
                {
                    if (!act)
                        prefix = prefix * 10 +  (a/pow10[j])%10 + (((a/pow10[j])%10>=c&&c!=0)?-1:0);
                    else
                        prefix = prefix * 10 +  9;

                }

            }
            total+=prefix+1;

        }
    }

    return total;
}
ifstream fin("cifre.in");
ofstream fout("cifre.out");
int main()
{
    long long a,b,c,k,maxNR=0,otherwise;

    fin >> a >> b >> c >> k;

    if (((till(b,c,k) -till(a-1,c,k))*10000/(b-a+1) )*1.0/10000 == ((till(b,c,k) -till(a-1,c,k))*10000/(b-a+1) )/10000)
    fout << ((till(b,c,k) -till(a-1,c,k))*10000/(b-a+1) )*1.0/10000<<".0000";
    else
    fout << ((till(b,c,k) -till(a-1,c,k))*10000/(b-a+1) )*1.0/10000;
}