#include<iostream.h>
#include<stdio.h>
FILE *f=fopen("cifre.in","r"),*g=fopen("cifre.out","w");
 int main()
{
long a,b,i;
float prob;
int k,c,nr1=0;
fscanf(f,"%ld %ld %d %d",&a,&b,&c,&k);
for(i=a;i<=b;i++)
{
int nr=0;
while(i)
{
if(i%10==c) nr++;
i/=10;
}
if(nr==k) nr1++;
}
prob=nr1/(float)(b-a);
fprintf(g,"%f",prob);
return 0;
}