#include <bits/stdc++.h>
using namespace std;
const int teta=1005000;
int dig(int num, int dig)
{
    int ans = 0;
    do{
        if(num%10==dig)
        ++ans;
        num/=10;
      }while(num);
    return ans;
}
int main()
{
    freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);
    srand(time(NULL));
    int a,b,c,k,t,e=0;
    scanf("%d%d%d%d",&a,&b,&c,&k);
    for(int i=1;i<=teta;++i)
    {
        t=a+rand()%(b-a+1);
        if(dig(t,c)>=k) ++e;
    }
    printf("%.4f\n",double(e)/teta);
}