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

long a,b,c,k,inc,nr;

float numara(long x)
{
	float nrc=0;
	while(x)
	{
		
		if(x%10==c)	
			nrc++;
		x /= 10;
	}
	return nrc;
}

float cauta()
{
	long i;
	float nr=0;
	for(i=a;i<=b;i++)
		if(numara(i)>=k)
			nr++;
	return nr;
}

float prob;

FILE *fin, *fout;

int main()
{
	fin = fopen("cifre.in", "rt");
	fout = fopen("cifre.out", "wt");
	fscanf(fin, "%ld%ld%ld%ld", &a, &b, &c, &k);
	prob=cauta()/ (b-a+1);
	fprintf(fout, "%.4f", prob);
	fclose(fin);
	fclose(fout);
	return 0;
}