#include <stdio.h>
#include <stdlib.h>
#include <time.h>
using namespace std;
int main()
{
	int sol,A,B,C,K,cnt,x,Timp;

	freopen("cifre.in","r",stdin);

	scanf("%d%d%d%d",&A,&B,&C,&K);

	Timp=1005000;

	srand(time(NULL));

	sol=0;
	while(Timp)
	{
		x=A+rand()%(B-A+1);

		cnt=0;
		while(x)
		{
			if(x%10==C) cnt++;
			x/=10;
		}

		if(cnt>=K) sol++;

		Timp--;
	}

	freopen("cifre.out","w",stdout);

	printf("%.4f",(double)sol/1005000);

	return 0;
}