#include<stdio.h>

#define input "cifre.in"
#define output "cifre.out"

int verifica(int n);

int a, b, c, k;

int main()
{
	FILE *in, *out;

	in = fopen (input, "r");
	out = fopen (output, "w");

	int cont=0, i;

	fscanf(in, "%d%d%d%d", &a, &b, &c, &k);

	for(i=a; i<=b; ++i)

		if(verifica(i))

			cont ++;

	float rez = (float) cont/(b-a+1) ;

	fprintf(out, "%.4f", rez);

	return 0;

}

int verifica(int n)
{
	int ind = 0;

	while(n)
	{
		if(n%10==c)

			++ind;

		n/=10;

		if(ind == k)

			return 1;

	}

	return 0;

}