#include <fstream>
#include <cstdio>
using namespace std;

long long v[10][10],a,b,k,C;

ifstream in("cifre.in");

long long inv(long long x)
{
	long long q;
	for (q=0;x;q=q*10+x%10,x/=10);
	return q;
}

long long ncif(long long x)
{
	long long p;
	for (p=0;x;x/=10,p++);
	return p;
}

inline long long max(long long a,long long b)
{
	return a>b ? a:b;
}

long long nr(long long x)
{
	long long rez=0,nr=C,q=ncif(x)-1;
	for (x=inv(x);x;x/=10,q--)
	{
		if (x%10<=k)
			rez+=(x%10+1)*v[q][max(nr,0)];
		else
			rez+=max(x%10-1,0)*v[q][max(nr,0)]+v[q][max(nr-1,0)];
		nr-=(x%10==k);
	}
	return rez;
}

int main()
{
	v[0][0]=1;
	for (long long i=1;i<10;i++)
		for (long long j=i;j>=0;j--)
			v[i][j]=v[i-1][j-1]+v[i-1][j]*9;
	for (long long i=1;i<10;i++)
		for (long long j=i;j>=0;j--)
			v[i][j]+=v[i][j+1];
	in>>a>>b>>k>>C;
	freopen("cifre.out","w",stdout);
	printf("%.4lf\n",(double)(nr(b)-nr(a-1))/(b-a+1));
	return 0;
}