#include<stdio.h>
#include<math.h>

int main(){
freopen("cifre.in","r",stdin);
freopen("cifre.out","w",stdout);
long a,b,c,k,nr=0,i,x,na,uc;
double pr;
scanf("%ld%ld%ld%ld",&a,&b,&c,&k);
for(i=a;i<=b;++i){
	x=i;na=0;
	do{
		uc=x%10;x/=10;
		if(uc==c) na++;
		}while(x&&na<k);
	if(na==k) {/*printf("%ld\n",i);*/nr++;}
	}
pr=(double)(nr)/(b-a+1);
printf("%0.4lf",pr);
return 0;
}