#include<stdio.h>
#include<math.h>

FILE *f = fopen("cifre.in","r");
FILE *g = fopen("cifre.out","w");

int A,B,C,K,Sol;

void citire(void)
{
    fscanf(f,"%d %d %d %d",&A,&B,&C,&K);
}

inline int nrCifre(int A,int K)
{
    int nr = (A%10 == K);
    A /= 10;

    for(;A;A/=10)
        nr += (A%10 == K);

    return nr;
}

void Rezolvare(void)
{
    for(int i=A;i<=B;i++)
        Sol += (nrCifre(i,C) >= K);
}

int main()
{
    citire();
    Rezolvare();

    fprintf(g,"%lf\n",(double)Sol/(B-A+1));
}