#include <bits/stdc++.h>
using namespace std;
ifstream f ("cifre.in");
ofstream g ("cifre.out");
int main()
{
    int sol,a,b,c,k,cnt,x,nr;
    f>>a>>b>>c>>k;
    nr=1005000;
    srand(time(NULL));
    sol=0;
    while(nr)
    {
        x=a+rand()%(b-a+1);
        cnt=0;
        while(x)
        {
            if(x%10==c) ++cnt;
            x/=10;
        }
        if(cnt>=k) ++sol;
        --nr;
    }
    g<<fixed<<setprecision(4)<<(double)sol/1005000;
    return 0;
}