Problem: While Grisha was celebrating New Year with Ded Moroz, Misha gifted Sasha a small rectangular pond of size n × m, divided into cells of size 1 × 1, inhabited by tiny evil fishes (no more than one fish per cell, otherwise they'll strife!).

The gift bundle also includes a square scoop of size r × r, designed for fishing. If the lower-left corner of the scoop-net is located at cell (x, y), all fishes inside the square (x, y)...(x + r - 1, y + r - 1) get caught. Note that the scoop-net should lie completely inside the pond when used.

Unfortunately, Sasha is not that skilled in fishing and hence throws the scoop randomly. In order to not frustrate Sasha, Misha decided to release k fishes into the empty pond in such a way that the expected value of the number of caught fishes is as high as possible. Help Misha! In other words, put k fishes in the pond into distinct cells in such a way that when the scoop-net is placed into a random position among (n - r + 1)·(m - r + 1) possible positions, the average number of caught fishes is as high as possible.

Input

The only line contains four integers n, m, r, k (1 ≤ n, m ≤ 105, 1 ≤ r ≤ min(n, m), 1 ≤ k ≤ min(n·m, 105)).

Output

Print a single number — the maximum possible expected number of caught fishes.

You answer is considered correct, is its absolute or relative error does not exceed 10 - 9. Namely, let your answer be a, and the jury's answer be b. Your answer is considered correct, if <image>.

Examples

Input

3 3 2 3


Output

2.0000000000


Input

12 17 9 40


Output

32.8333333333

Note

In the first example you can put the fishes in cells (2, 1), (2, 2), (2, 3). In this case, for any of four possible positions of the scoop-net (highlighted with light green), the number of fishes inside is equal to two, and so is the expected value.

<image>

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
template <typename T>
T in() {
  char ch;
  T n = 0;
  bool ng = false;
  while (1) {
    ch = getchar();
    if (ch == '-') {
      ng = true;
      ch = getchar();
      break;
    }
    if (ch >= '0' && ch <= '9') break;
  }
  while (1) {
    n = n * 10 + (ch - '0');
    ch = getchar();
    if (ch < '0' || ch > '9') break;
  }
  return (ng ? -n : n);
}
template <typename T>
inline T POW(T B, T P) {
  if (P == 0) return 1;
  if (P & 1)
    return B * POW(B, P - 1);
  else
    return (POW(B, P / 2) * POW(B, P / 2));
}
template <typename T>
inline T Gcd(T a, T b) {
  if (a < 0) return Gcd(-a, b);
  if (b < 0) return Gcd(a, -b);
  return (b == 0) ? a : Gcd(b, a % b);
}
template <typename T>
inline T Lcm(T a, T b) {
  if (a < 0) return Lcm(-a, b);
  if (b < 0) return Lcm(a, -b);
  return a * (b / Gcd(a, b));
}
long long Bigmod(long long base, long long power, long long MOD) {
  long long ret = 1;
  while (power) {
    if (power & 1) ret = (ret * base) % MOD;
    base = (base * base) % MOD;
    power >>= 1;
  }
  return ret;
}
bool isVowel(char ch) {
  ch = toupper(ch);
  if (ch == 'A' || ch == 'U' || ch == 'I' || ch == 'O' || ch == 'E')
    return true;
  return false;
}
template <typename T>
long long int isLeft(T a, T b, T c) {
  return (a.x - b.x) * (b.y - c.y) - (b.x - c.x) * (a.y - b.y);
}
long long ModInverse(long long number, long long MOD) {
  return Bigmod(number, MOD - 2, MOD);
}
bool isConst(char ch) {
  if (isalpha(ch) && !isVowel(ch)) return true;
  return false;
}
int toInt(string s) {
  int sm;
  stringstream second(s);
  second >> sm;
  return sm;
}
struct z {
  long long int vl;
  long long int x, y;
  z(){};
  z(long long int a, long long int b, long long int c) { vl = a, x = b, y = c; }
  bool operator<(const z &p) const { return (vl < p.vl); }
};
long long int n, m, r, k;
long long int N, M;
long long int koto(long long int x, long long int y) {
  long long int l = min(x, r);
  long long int rg = min(y, r);
  long long int ans = l * rg;
  int f = 0;
  if (y > M) {
    ans -= l * (y - M);
    f++;
  }
  if (x > N) {
    ans -= rg * (x - N);
    f++;
  }
  if (f == 2) {
    ans += (y - M) * (x - N);
  }
  return ans;
}
map<pair<int, int>, int> mp;
int dx[] = {0, 0, -1, 1, -1, -1, 1, 1};
int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
int main() {
  priority_queue<z> q;
  priority_queue<z> qq;
  cin >> n >> m >> r >> k;
  N = n - r + 1;
  M = m - r + 1;
  q.push(z(koto(N, M), N, M));
  long long int tot = N * M;
  long long int ans = 0;
  long long int nw = k;
  while (q.size() && nw--) {
    z tp = q.top();
    q.pop();
    qq.push(tp);
    mp[make_pair(tp.x, tp.y)] = 1;
    for (int i = 0; i < 8; i++) {
      long long int x = tp.x + dx[i];
      long long int y = tp.y + dy[i];
      if (x > 0 && y > 0 && x <= n && y <= m) {
        if (mp.find(make_pair(x, y)) == mp.end()) {
          q.push(z(koto(x, y), x, y));
          mp[make_pair(x, y)] = 1;
        }
      }
    }
  }
  while (qq.size() && k--) {
    z tp = qq.top();
    qq.pop();
    ans += tp.vl;
  }
  double an = (ans * 1.0) / (tot * 1.0);
  printf("%.12lf\n", an);
  return 0;
}