#include <math.h>
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("cifre.in");
ofstream out("cifre.out");
int cifre(long n, int b)
{
    int a;
    a = 0;
    while(n!=0)
    {
        if(n%10 == b)
        {
            a++;
        }
        n/=10;
    }
    return a;
}
int main()
{
    out << setprecision(4);
    long a,b,k,c,l,r;
    in >> a;
    in >> b;
    in >> c;
    in >> k;
    in.close();
    l=(b-a+1);
    r=0;
    while(a<=b)
    {

        if(cifre(a, c) >= k)
        {
            r++;
        }
        a++;
    }
    out<<((double) r) / l;
return 0;
}