#include <fstream>
#include <vector>
#include <iomanip>
 
using namespace std;
 
ifstream fin("cifre.in");
ofstream fout("cifre.out");
 
const int xmax= 1000000000;
const int pl= 10000;// piece length, aprox. sqrt(xmax);
const int f[6]= {10000, 3439, 523, 36, 1, 0};
const int output_precision= 4;
 
int c_cnt(int x, int c){
    int sol= 0;
    while (x>0){
        if (x%10==c){
            ++sol;
        }
        x/= 10;
    }
    return sol;
}
 
int main(){
    int a, b, c, k;
    fin>>a>>b>>c>>k;
    int dif= b-a+1;
 
    int sol= 0;
    if (b-a+1>pl){
        while (a%pl!=0){
            if (c_cnt(a, c)>=k){
                ++sol;
            }
            ++a;
        }
        while (b-a+1>pl){
            int aux= k-c_cnt(a/pl, c);
            if (aux<0){
                aux= 0;
            }else if (aux>5){
                aux= 5;
            }
            sol+= f[aux];
            a+= pl;
        }
    }
    while (a<=b){
        if (c_cnt(a, c)>=k){
            ++sol;
        }
        ++a;
    }
    fout<<setprecision(output_precision)<<fixed;
    fout<<(double)sol/dif<<"\n";
 
    return 0;
}