#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

int Sub(int nr, int c)
{ int k=0;
     do{
      if (c==nr%10) k++;
      nr/=10;}while(nr);
      return k;

}

int main()
{long long int A,B;
    int C,K;
  int i, save, nr, imp=0, cif,de;
    ifstream f("cifre.in");
    ofstream g("cifre.out");
    f>>A>>B>>C>>K;
    int l=1; save=K;

    do{l*=10;
        save--;
        }while(save);
        l=l/10;
        if (l>A) A=l;

    for (i=A;i<=B;i++) { nr=Sub(i,C);
                        if (nr>=K) imp++;}

    de=B-A+1;
    g<<fixed << setprecision(4) <<(float)imp/de;
    return 0;
}