#include <iostream>
#include <fstream>
using namespace std;
long i,y,nr,A,B,C,K,c,ci;
int main()
{fstream fin,fout;
fin.open("cifre.in",ios::in);
fout.open("cifre.out",ios::out);
fin>>A;
fin>>B;
fin>>C;
fin>>K;
y=0;
for (i=A;i<=B;i=i++){nr=0;ci=i;
    while (ci!=0){c=ci%10;if(c==C)nr=nr+1;ci=ci/10;};
if (nr>=K)y=y+1;
};fout<<(float)y/B<<" ";fout<<endl;
fin.close();
fout.close();
    return 0;
}