#include <math.h>
#include <iostream>
#include <fstream>
using namespace std;
int cifre(long n, long b)
{
    long a;
    a = 0;
    while(n!=0)
    {
        if(n%10 == b)
        {
            a++;
        }
        n/=10;
    }
    return a;
}
int main()
{
    fstream f;
    f.open("cifre.in", fstream::in);
    long a,b,k,c;
    float l = 0, r = 0;
    f >> a;
    f >> b;
    f >> c;
    f >> k;
    f.close();
    while(a<=b)
    {

        if(cifre(a, c) >= k)
        {
            r++;
        }
        l++;
        a++;
    }
    f.open("cifre.out", fstream::out);

    f<< floor(r/l * 10000) / 10000;
    return 0;
}