#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("cifre.in");
ofstream out("cifre.out");

const int LIM = 1e4;

int i, x, k, a, b, c, res;
int power[]={1, 9, 81, 729, 6561};
int comb[10][10];

int get_digits(int x) {
  int cnt=0;
  if(c == 0 && x == 0)
    return 1;

  while(x != 0) {
    if(x % 10 == c)
      cnt++;
    x /= 10;
  }

  return cnt;
}

int main()
{
  in >> a >> b >> c >> k;

  comb[4][0] = 1;
  comb[4][1] = 4;
  comb[4][2] = 6;
  comb[4][3] = 4;
  comb[4][4] = 1;

  int i;
  for(i = a; (i % LIM != 0 || i < LIM) && i <= b; i++)
    if(k <= get_digits(i))
      res ++;

  while(i + LIM <= b) {
    x = max(k - get_digits(i / LIM), 0);
    while(x<=4) {
      res += comb[4][x] * power[4 - x];
      x++;
    }
    i += LIM;
  }

  while(i <= b) {
    if(k <= get_digits(i))
      res++;
    i++;
  }

  double sol = (double)res / (b - a + 1);

  out << setprecision(10) << fixed;
  out << sol << '\n';

  in.close();
  out.close();
  return 0;
}