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

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

int main()
{
    int A,B,C,K;
    double cazfav=0,cazpos=0;
    double probabilitate;
    fin >> A >> B >> C >> K;
    cazpos = B - A+1;
    for(int i=A;i<=B;i++){
        bool are_kcifre = false;
        bool are_cifra = false;
        int nrcif=0,j;
        j=i;
        while(j!=0){
            nrcif++;
            if(j%10==C)
                are_cifra = true;
            j/=10;
        }
        if(nrcif>=K)
            are_kcifre = true;
        if(are_cifra && are_kcifre)
            cazfav++;
    }
    probabilitate = cazfav/cazpos;
    fout << probabilitate;
    return 0;
}