#include <cstdio>
using namespace std;

int a,b,c,k,x,i;
float rasp,nr,soll,nrap;
int main(){
		freopen("cifre.in","r",stdin);
		freopen("cifre.out","w",stdout);
		scanf("%d %d %d %d",&a,&b,&c,&k);
	for(i=a;i<=b;i++){
		x=i;nrap=0;
		while(x!=0){
			if(x%10==c){
				nrap++;
			}
			x/=10;
		}
		if(nrap>=k)
			rasp++;
		nr++;
	}
	soll=rasp/nr;
	printf("%1.4f",soll);
}