#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
void solve() {
  int n, m, r, k;
  cin >> n >> m >> r >> k;
  vector<int> w;
  vector<int> w1;
  int t = 0;
  for (int i = 0; i < n; i++) {
    if (i >= (n + 1) / 2) {
      if ((n % 2 == 0) && (i == n / 2)) t += 1;
      t -= 1;
    } else
      t += 1;
    int t1 = min(t, r);
    t1 = min(t1, n - r + 1);
    w1.push_back(t1);
  }
  t = 0;
  for (int i = 0; i < m; i++) {
    if (i >= (m + 1) / 2) {
      if ((m % 2 == 0) && (i == m / 2)) t += 1;
      t -= 1;
    } else
      t += 1;
    int t1 = min(t, r);
    t1 = min(t1, m - r + 1);
    w.push_back(t1);
  }
  set<pair<long long int, pair<int, int>>> e;
  set<pair<int, int>> done;
  int s = (n + 1) / 2 - 1;
  t = (m + 1) / 2 - 1;
  e.insert(make_pair((w1[s] * w[t]), make_pair(s, t)));
  done.insert(make_pair(s, t));
  long long int ans = 0;
  while (k > 0) {
    pair<int, pair<int, int>> er = *e.rbegin();
    long long int cur = er.first;
    int s = er.second.first;
    int t = er.second.second;
    ans += cur;
    k -= 1;
    e.erase(er);
    if (s > 0) {
      if (done.find(make_pair(s - 1, t)) == done.end()) {
        e.insert(make_pair((w1[s - 1] * w[t]), make_pair(s - 1, t)));
        done.insert(make_pair(s - 1, t));
      }
    }
    if (t > 0) {
      if (done.find(make_pair(s, t - 1)) == done.end()) {
        e.insert(make_pair((w1[s] * w[t - 1]), make_pair(s, t - 1)));
        done.insert(make_pair(s, t - 1));
      }
    }
    if (s < n - 1) {
      if (done.find(make_pair(s + 1, t)) == done.end()) {
        e.insert(make_pair((w1[s + 1] * w[t]), make_pair(s + 1, t)));
        done.insert(make_pair(s + 1, t));
      }
    }
    if (t < m - 1) {
      if (done.find(make_pair(s, t + 1)) == done.end()) {
        e.insert(make_pair((w1[s] * w[t + 1]), make_pair(s, t + 1)));
        done.insert(make_pair(s, t + 1));
      }
    }
  }
  long double tw = (double)(n - r + 1);
  long double t1 = (double)(m - r + 1);
  cout << fixed << setprecision(9) << ans / (tw * t1) << endl;
}
int main() {
  cin.sync_with_stdio(false), cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  int c = 0;
  t = 1;
  while (t--) {
    solve();
    c += 1;
  }
  return 0;
}
