#include <fstream>
#define L 10000

using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
long long x,p,n,a,b,k,c,nr;
int cifre(int n)
{
    int nr=0;
    for(; n; n/=10)
    {
        if(n%10==c)nr++;
    }
    return nr;
}
int main()
{
    f>>a>>b>>c>>k;
    if(b-a>=L)
    {
        for(; a%L; ++a,p++)
        {
            if(cifre(a)>=k)nr++;
        }
        for(x=0; b-a>=L; a+=L,p+=L)
        {
            x=cifre(a/L);
            if(x>=k)nr+=6561;
            if(x>=k-1)nr+=2916;
            if(x>=k-2)nr+=487;
            if(x>=k-3)nr+=35;
            if(x>=k-4)nr++;
        }
    }
    for(; a<=b; ++a,++p)
    {
        if(cifre(a)>=k)nr++;
    }
    g<<(double)nr/p;
    return 0;
}