#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
priority_queue<pair<long long, long long> > pq;
int n, m, r, k, row[N], col[N];
int id[N];
long long ans;
int main() {
  cin >> n >> m >> r >> k;
  for (int i = 1; i <= n; i++) {
    row[i] = min(min(i, n - i + 1), min(r, n - r + 1));
  }
  for (int i = 1; i <= m; i++) {
    col[i] = min(min(i, m - i + 1), min(r, m - r + 1));
  }
  sort(col + 1, col + m + 1);
  for (int i = 1; i <= n; i++) {
    id[i] = m;
    pq.push(make_pair(row[i] * col[m], i));
  }
  while (k--) {
    ans += pq.top().first;
    int cur = pq.top().second;
    pq.pop();
    pq.push({row[cur] * col[--id[cur]], cur});
  }
  cout << fixed << setprecision(10)
       << ans * 1.0 / ((n - r + 1) * 1ll * (m - r + 1)) << '\n';
  return 0;
}
