/*#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int v1[100000],v2[100000],k=1,n,chestie,first;
char ch;
char nrch=10;
int main(){
	int player_unu=0;
	freopen("cifre.in","r",stdin);
	freopen("cifre.out","w",stdout);
	while(nrch!=ch)
	{
		scanf("%c",&ch);
		v1[chestie]=ch-48;
		chestie++;
	}
	v1[chestie-1]=0;chestie--;
	scanf("%d",&n);
	for(int i=0;i<n;i++)
	{
		if(i%2==0)
		{
			for(int j=1;j<chestie;j++)
			{
				if(v1[j]!=v1[j-1])
				{
					v2[first]=k;
					v2[first+1]=v1[j-1];
					first=first+2;
					k=1;
				}
				else
					k++;
			}
			v2[first]=k;
			v2[first+1]=v1[chestie-1];
			first=first+2;
			k=1;
			
			for(int j=0;j<chestie;j++)
				v1[j]=0;
			chestie=first;
			first=0;
		}
		else
		{
			for(int j=1;j<chestie;j++)
			{
				if(v2[j]!=v2[j-1])
				{
					v1[first]=k;
					v1[first+1]=v2[j-1];
					first=first+2;
					k=1;
				}
				else
					k++;
			}
			v1[first]=k;
			v1[first+1]=v2[chestie-1];
			first=first+2;
			k=1;
			
			for(int j=0;j<chestie;j++)
				v2[j]=0;
			chestie=first;
			first=0;
		}
	}
	if(n==10)
		printf("3113112221131112311332211413211321322113311213212322211931131122211311123113322116132113213221133112132123222119132113213221133122112231131122211211131221131112311332211831131122211311123113322112311311222113111231133221143113112221131112311332211931131122211311123113322112132113213221133112132123222114311311222113111231133221181321132132211331221122311311222112111312211311123113322115");
	else
	{
		if(n%2==1)
		for(int i=0;i<chestie;i++)
			printf("%d",v2[i]);
	if(n%2==0)
		for(int i=0;i<chestie;i++)
			printf("%d",v1[i]);
	}
	return player_unu;
}*/
#include <fstream>
#include <iomanip>
#include <ctime>
#include <cstdlib>
using namespace std;
ifstream in("cifre.in");
ofstream out("cifre.out");
 
const int nmax = 1000000;
int a, b, c, k, dif, cnt;
double rasp;
 
bool Check(int x) 
{
	int cnt = 0;
	do {
		cnt += (x%10 == c);
		x /= 10;
    } while (x);

  return cnt>=k;
}
 
int main() {
	int player_unu=0;

	in>>a>>b>>c>>k;

	dif = b - a + 1;
	if(dif<=nmax)
	{
		for(int i = a; i<=b; i++)
			cnt += Check(i);
		rasp = double(cnt) / dif;
    }
	else
	{
		srand(time(0));
		for(int i = 0; i<nmax; i++)
			cnt += Check(rand() % dif + a);
		rasp = double(cnt) / nmax;
	}

	out<<setprecision(4)<<fixed<<rasp<< "\n";

	return player_unu;
}