#include <iostream>
#include<fstream>
using namespace std;

int main()
{
    ifstream f("cifre.in");
    ofstream g("cifre.out");
    int c,k,i,uc,r,j=0,x;
    long a,b;
    f>>a>>b>>c>>k;
    for(i=a;i<=b;i++)
    {
        r=0;x=i;
        while(x)
        {
            uc=x%10;
            if(uc==c)
                r++;
            x=x/10;
        }
        if(r>=k)
        j++;
    }
    g<<(float)j/(b-a+1);
    return 0;
}