#include <bits/stdc++.h>
using namespace std;
void readi(int &x) {
  int v = 0, f = 1;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = v * 10 + c - '0';
  while (isdigit(c = getchar())) v = v * 10 + c - '0';
  x = v * f;
}
void readll(long long &x) {
  long long v = 0ll, f = 1ll;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = v * 10 + c - '0';
  while (isdigit(c = getchar())) v = v * 10 + c - '0';
  x = v * f;
}
void readc(char &x) {
  char c;
  while ((c = getchar()) == ' ')
    ;
  x = c;
}
void writes(string s) { puts(s.c_str()); }
void writeln() { writes(""); }
void writei(int x) {
  if (!x) putchar('0');
  char a[25];
  int top = 0;
  while (x) {
    a[++top] = (x % 10) + '0';
    x /= 10;
  }
  while (top) {
    putchar(a[top]);
    top--;
  }
}
void writell(long long x) {
  if (!x) putchar('0');
  char a[25];
  int top = 0;
  while (x) {
    a[++top] = (x % 10) + '0';
    x /= 10;
  }
  while (top) {
    putchar(a[top]);
    top--;
  }
}
long long qp(long long x, long long y) {
  if (y == 0) return 1;
  if (y == 1) return x;
  long long z = qp(x, y / 2);
  z = z * z % 1000000007;
  if (y & 1) z = z * x % 1000000007;
  return z;
}
long long n, m, r, k, i, j, x, y;
long long dx[4] = {-1, 1, 0, 0}, dy[4] = {0, 0, -1, 1};
double ans;
string st;
set<pair<long long, long long> > vis;
bool operator<(pair<long long, pair<long long, long long> > x,
               pair<long long, pair<long long, long long> > y) {
  if (x.first != y.first) return x.first > y.first;
  return x.second < y.second;
}
priority_queue<pair<long long, pair<long long, long long> > > pq;
long long ss(long long ii, long long jj) {
  return min(min(ii, n - ii + 1), min(r, n - r + 1)) *
         min(min(jj, m - jj + 1), min(r, m - r + 1));
}
int main() {
  ios::sync_with_stdio(false);
  ;
  cin >> n >> m >> r >> k;
  while (!pq.empty()) pq.pop();
  pq.push(make_pair(ss((n + 1) / 2, (m + 1) / 2),
                    make_pair((n + 1) / 2, (m + 1) / 2)));
  vis.insert(make_pair((n + 1) / 2, (m + 1) / 2));
  while (k--) {
    ans += pq.top().first;
    x = pq.top().second.first;
    y = pq.top().second.second;
    pq.pop();
    for (i = 0; i < 4; i++) {
      int xx = x + dx[i], yy = y + dy[i];
      if (xx < 1 || yy < 1 || xx > n || yy > m) continue;
      if (vis.count(make_pair(xx, yy))) continue;
      vis.insert(make_pair(xx, yy));
      pq.push(make_pair(ss(xx, yy), make_pair(xx, yy)));
    }
  }
  printf("%.12f", ans / ((double)n - r + 1) / ((double)m - r + 1));
  return 0;
}
