#include <iostream>
#include <fstream>
using namespace std;
int A,B,C,K,i,x,s,S,k1;
double k;
int main()
{
    ifstream f("cifre.in");
    ofstream g("cifre.out");
    f>>A>>B>>C>>K;
    for (i=A;i<=B;i++)
    {
        x=i;
        while (x!=0)
        {
            if (x%10==C&&s<=K) s++;
            x/=10;
        }
        if (s>=K) S++;
        s=0;
    }
    k=(double)S/(B-A+1);
    k*=10000;
    k1=k;
    k=(double)k1/10000;
    g<<k;
    return 0;
}