#include<stdio.h>


int cf(int n,int c)
{
int nr=0;
while(n>0)
{
if(n%10==c) nr++;
n/=10;
}

return nr;
}

int main()
{
int A,B,i,c,K,nr=0;
float rez;
FILE *f=fopen("cifre.in","r");

fscanf(f,"%i %i %i %i",&A,&B,&c,&K);


for(i=A;i<=B;i++)
if(cf(i,c)>=K) nr++;

rez=(float) nr/(B-A+1);
fclose(f);
f=fopen("cifre.out","w");
fprintf(f,"%.4f",rez);
fclose(f);
return 0;
}