#include<stdio.h>
int a,b,c,k,itv,nr,sol,x,i;

int main(){
	FILE*f=fopen("cifre.in","r");
	FILE*g=fopen("cifre.out","w");
	
	fscanf(f,"%d %d %d %d",&a,&b,&c,&k);
	
	itv=b-a+1;
	for(i=1;i<=itv;i++){
		x=a+i;
		nr=0;
		while(x){
			if(x%10==c)
				nr++;
			x/=10;
		}
		if(nr>=k)
			sol++;
	}
	
	fprintf(g,"%.4lf",(double)sol/itv);
	
	fclose(f);
	fclose(g);
	return 0;
}