#include <stdio.h>
#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pdd pair<ld, ld>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

int a, b, c, k;
int dp[15][15][3];
int v[15];

int Solve(int x) {
  if (x < 0) {
    return 0;
  }

  if (x == 0) {
    return (c == 0 && k <= 1);
  }

  // dp[i][j][0/1/2] = length i, j digits == c, 0/1/2 - less/equal/bigger than x
  //
  memset(v, 0, sizeof(v));
  memset(dp, 0, sizeof(dp));
  int n = 0;
  do {
    v[++n] = x % 10;
    x /= 10;
  } while (x);
  reverse(v + 1, v + n + 1);

  for (int i = 1; i < v[1]; i++) {
    dp[1][(i == c)][0]++;
  }
  dp[1][(v[1] == c)][1] = 1;
  for (int i = v[1] + 1; i < 10; i++) {
    dp[1][(i == c)][2]++;
  }

  for (int i = 1; i < n; i++) {
    for (int j = 0; j <= n; j++) {
      for (int k = 0; k < 10; k++) {
        if (k < v[i + 1]) {
          dp[i + 1][j + (k == c)][0] += dp[i][j][0] + dp[i][j][1];
          dp[i + 1][j + (k == c)][2] += dp[i][j][2];
        } else if (k == v[i + 1]) {
          dp[i + 1][j + (k == c)][0] += dp[i][j][0];
          dp[i + 1][j + (k == c)][1] += dp[i][j][1];
          dp[i + 1][j + (k == c)][2] += dp[i][j][2];
        } else {
          dp[i + 1][j + (k == c)][0] += dp[i][j][0];
          dp[i + 1][j + (k == c)][2] += dp[i][j][1] + dp[i][j][2];
        }
      }
    }
  }

  int ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = k; j <= n; j++) {
      ans += dp[i][j][0] + dp[i][j][1];
      if (i != n) {
        ans += dp[i][j][2];
      }
    }
  }

  return ans;
}

int main() {
  cin.sync_with_stdio(false);

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

  cin >> a >> b >> c >> k;
  cout << fixed << setprecision(6)
       << 1.0 * (Solve(b) - Solve(a - 1)) / (b - a + 1);

  return 0;
}