#include<cstdio>
using namespace std;
long long int i,a,b,m,c,k;
double n,j,v,p;
int main(){
	freopen("cifre.in","r",stdin);
	freopen("cifre.out","w",stdout);
	scanf("%lld %lld",&a,&b);
	scanf("%lld %lld",&c,&k);
	
	for(i=a;i<=b;i++){
		m=i;j=0;
		while(m!=0){
			if(m%10==c)
				j++;
			m/=10;}
		if(j>=k)
			n++;
		v++;}
	p=n/v;
	printf("%1.4f",p);
	return 0;}