#include <iostream>
#include <fstream>
#include <iomanip>
#define MAX 504
using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
long long int a,b,k,c,i,u,nr,p;
double t,t1,t2;
int main()
{
   cin>>a>>b>>c>>k;
   nr=b-a+1;
   for(i=a;i<=b;i++)
   {   int x=i;
       u=0;
       while( x&& u!=k)
       {
           if(x%10==c) u++;
           x/=10;
       }

       if(u>=k) p++;
   }
   t1=p;
   t2=nr;
   t=t1/t2;
   cout<<setprecision(4)<<t;
}