#include<fstream>

using namespace std;
long long x,p,n,a,b,k,c;
long long nr;
int cifre(int n)
{
    int nr=0;
    while(n)
    {
        if(n%10==c)
        nr++;
        n/=10;
    }
    return nr;
}
int main()
{
    ifstream f("cifre.in");
    ofstream g("cifre.out");
    f>>a>>b>>c>>k;
    if(b-a>=10000)
    {
        while(a%10000)
        {
            if(cifre(a)>=k)
            nr++;
            a++;
            p++;
        }
        for(x=0; b-a>=10000; a+=10000,p+=10000)
        {
            x=cifre(a/10000);
            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++;
        }
    }
    while(a<=b)
    {
        if(cifre(a)>=k)
        nr++;
        a++;
        p++;
    }
    g<<(double)nr/p;
    return 0;
}