#include <fstream>

using namespace std;

int main()
{
    int c, k, A, B, nr,  i, j, s;
    double dif, posibilitate;
    ifstream in("cifre.in");
    ofstream out("cifre.out");
    in >> A >> B >> c >> k;
    posibilitate=nr=s=0;
    dif=0;
    for(i=A;i<=B;i++)
    {
    j=i;
    s=0;
    while(s!=k&&j!=0)
    { if(j%10==c)s++;
      j=j/10;
      if(s>=k)nr++;
    }
    dif++;
    }
    in.close();
    posibilitate=nr/dif/1.0;
    out << posibilitate <<"\n";
    out.close();
    return 0;
}