#include <fstream>
#include <iostream>

using namespace std;
ifstream fin("cifre.in");
ofstream fout("cifre.out");

int verificare_cifre(int n,int c,int k)
{
    int ct=0;
    while(n!=0)
    {
        if(n%10==c)
            ct++;
        n/=10;
    }
    if(ct>=k)
        return 1;
    else
        return 0;
}

int main()
{
    long long A,B;
    int C,K;
    double p;
    int nr=0;
    fin>>A>>B>>C>>K;
    for(int i=A;i<=B;i++)
    {
        if(verificare_cifre(i,C,K)==1)
            nr++;
    }
    p=(double)nr/(B-A+1);
    p=(int)(p*10000);
    p=(double)(p/10000);
    fout<<p;
    return 0;
}