#include<bits/stdc++.h>

#define forn(i, n) for (int i = 0 ; i < int(n) ; i++)

#define fore(i, s, t) for (int i = s ; i < (int)t ; i++)

#define fi first

#define se second

#define all(x) x.begin(),x.end()

#define pf2(x,y) printf("%d %d\n",x,y)

#define pf(x) printf("%d\n",x)

#define each(x) for(auto it:x)  cout<<it<<endl;

#define pii pair<int,int>

using namespace std;

typedef long long ll;

const int maxn=2e5+5;

const int maxm=2e5+5;

const int inf=1e9;

int n,m,r,k;

struct cell{

	int x,y,d;

	ll val;

	cell(int x,int y,int d):x(x),y(y),d(d){

		val=(min(m+1,y+r)-max(y,r))*1ll*(min(n+1,x+r)-max(x,r));

	}

	bool operator<(const cell& rhs)const {

		return val>rhs.val;

	}

};

multiset<cell> ms;

double calc(){

	for (int i = 1; i <= n; i++) {

		int j = m / 2;

		ms.insert({ i, j, 0 });

		ms.insert({ i, j + 1, 1 });

	}

	ll total = 0;

	for(int i=0;i<k;i++){

		auto it = *ms.begin();

		total += it.val;

		ms.erase(ms.begin());

		if (it.d == 1 && it.y < m)

			ms.insert({ it.x, it.y + 1, 1 });

		else if (it.d == 0 && it.y > 1)

			ms.insert({ it.x, it.y - 1, 0 });

	}

	return (double)total / ((n - r + 1) * 1LL * (m - r + 1));

}

int main(){

	cin>>n>>m>>r>>k;

	if(n>m) swap(n,m);

	if(m==1) {

		printf("%.10f\n",1.0);

	}

	else {

		printf("%.10f\n",calc());

	}

}