1 #include<bits/stdc++.h>
 2 using namespace std;
 3 const int MAX_N = 1e5+9;
 4 long long N,M,r,k;
 5 struct P
 6 {
 7     long long first,second;
 8     P(int x,int y){first = x,second = y;}
 9 };
10 priority_queue <P> que;
11 set<int> st[MAX_N];
12 long long get_val(P t)
13 {
14     return (min(N - r + 1, t.first) - max(1ll, t.first - r + 1) + 1) * (min(M - r + 1, t.second) - max(1ll, t.second - r + 1) + 1);
15 }
16 bool operator < (const P &a,const P &b)
17 {
18     return get_val(b) > get_val(a);
19 }
20
21 int main()
22 {
23     while(cin>>N>>M>>r>>k)
24     {
25         while(!que.empty()) que.pop();
26         for(int i=0;i<MAX_N;i++) st[i].clear();
27         int x = (N+1)/2;
28         int y = (M+1)/2;
29         que.push(P(x,y));
30         st[x].insert(y);
31         double ans = 0;
32         long long num = 0;
33         while(!que.empty())
34         {
35             P t = que.top();que.pop();
36             ans += (get_val(t)*1.0)/((N-r+1)*(M-r+1)*1.0);
37             k--;
38             if(!k) break;
39             //cout<<t.first<<"..."<<t.second<<"...."<<get_val(t)<<endl;
40             if(t.first+1>0 && t.first+1<=N && st[t.first+1].count(t.second) == 0) que.push(P(t.first+1,t.second)),st[t.first+1].insert(t.second);
41             if(t.first-1>0 && t.first-1<=N && st[t.first-1].count(t.second) == 0) que.push(P(t.first-1,t.second)),st[t.first-1].insert(t.second);
42             if(t.second-1 >0 && t.second-1 <=M && st[t.first].count(t.second-1) == 0) que.push(P(t.first,t.second-1)),st[t.first].insert(t.second-1);
43             if(t.second+1 >0 && t.second+1 <=M && st[t.first].count(t.second+1) == 0) que.push(P(t.first,t.second+1)),st[t.first].insert(t.second+1);
44         }
45         printf("%.10lf\n",ans);
46     }
47     return 0;
48 }