#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int A,B,k,c,i,count;
double procent;
char numar[12],string[12];

int main()
{
	FILE * in = fopen("cifre.in","rt");
	FILE * out = fopen("cifre.out","wt");
	
	fscanf(in,"%d",&A);
	fscanf(in,"%d",&B);
	fscanf(in,"%d",&k);
	fscanf(in,"%d",&c);
	
	for(i=0;i<k;i++)
		numar[i] = c+48;
	
	for(i=A;i<=B;i++)
	{
		itoa(i,string,10);
		
		if(strstr(string,numar))
			count++;
	}
	
	procent = (double) count / (B - A + 1);
	
	//printf("%d\n",count);
	
	fprintf(out,"%lf",procent);
	
	return (0);
}