#include <stdio.h>

long long a, b;
int c, k;
long long aux, i, nr;
long int sol;
float rez;

int cifre (long long x)
{
	aux = x;
	nr = 0;
	while (aux)
	{
		if (aux % 10 == c)
			nr ++;
		aux /= 10;
	}
	return nr;
}

int main()
{
	FILE *f = fopen ("cifre.in","r");
	FILE *g = fopen ("cifre.out","w");
	fscanf (f,"%lld %lld %lld %lld", &a, &b, &c, &k);
	fclose(f);
	
	for (i=a; i<=b; ++i)
	{
		if (cifre (i) >= k)
			sol ++;
	}
	
	rez = (float) sol / (b-a+1);
	fprintf (g,"%.4f", rez);
	
	fclose(g);
	return 0;
}