#include <bits/stdc++.h>
using namespace std;
int main(int argc, char *argv[]) {
  int n, m, r, k;
  cin >> n >> m >> r >> k;
  int dx[] = {-1, 0, 1, 0};
  int dy[] = {0, -1, 0, 1};
  priority_queue<tuple<double, int, int>> q;
  double S = (long long)(n - r + 1) * (m - r + 1);
  auto cal = [&](int x, int y) {
    double w = min(x, n - r) - max(0, x - r + 1) + 1;
    double h = min(y, m - r) - max(0, y - r + 1) + 1;
    return w * h / S;
  };
  set<pair<int, int>> se;
  q.push({cal(n / 2, m / 2), n / 2, m / 2});
  se.insert({n / 2, m / 2});
  double ans = 0;
  while (k--) {
    double v;
    int x, y;
    tie(v, x, y) = q.top();
    q.pop();
    ans += v;
    for (int i = 0; i < 4; i++) {
      int tx = x + dx[i], ty = y + dy[i];
      if (!se.count({tx, ty})) {
        se.insert({tx, ty});
        q.push({cal(tx, ty), tx, ty});
      }
    }
  }
  printf("%.10f\n", ans);
  return 0;
}
