#include<fstream>
#include<cmath>
#include<iomanip>
using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
int verificare(int x,int c,int k)
{
    int i,ap=0,prob=0;
    while(x!=0)
    {
        if(x%10==c) ap++;
        x/=10;
    }
    if(ap>=k) prob=1;
    return prob;
}
int main()
{
    int k,c,s=0;
    long long a,i,b;
    f>>a>>b>>c>>k;
    for(i=a;i<=b;i++)
    {
        s+=verificare(i,c,k);
    }
    double cs;
    cs=(float)s/(b-a+1);
    g<<setprecision(4)<<cs;
    return 0;
}