bzoj1132

 1 const oo=1 shl 30;
 2       eps=1e-8;
 3 var i,j,k,m,n:longint;
 4     x,y:array[0..6010] of longint;
 5     z:array[0..6010] of double;
 6     ans,xx,yy:int64;
 7 
 8 procedure swap(var a,b:longint);
 9   var c:longint;
10   begin
11     c:=a;
12     a:=b;
13     b:=c;
14   end;
15 
16 procedure sort(l,r:longint);
17   var i,j:longint;
18       p,q:double;
19   begin
20     i:=l; j:=r;
21     p:=z[(l+r) shr 1];
22     repeat
23       while z[i]<p-eps do inc(i);
24       while z[j]>p+eps do dec(j);
25       if i<=j then
26       begin
27         swap(x[i],x[j]);
28         swap(y[i],y[j]);
29         q:=z[i]; z[i]:=z[j]; z[j]:=q;
30         inc(i); dec(j);
31       end;
32     until i>j;
33     if l<j then sort(l,j);
34     if i<r then sort(i,r);
35   end;
36 
37 begin
38   readln(n);
39   for i:=1 to n do
40     readln(x[i],y[i]);
41   for i:=1 to n-2 do
42   begin
43     k:=i;
44     for j:=i to n do
45       if x[j]<x[k] then k:=j;
46     swap(x[i],x[k]);
47     swap(y[i],y[k]);
48     for j:=i+1 to n do
49       if x[j]=x[i] then
50         if y[j]>y[i] then z[j]:=oo
51         else z[j]:=-oo
52       else z[j]:=(y[j]-y[i])/(x[j]-x[i]);
53     sort(i+1,n);
54     xx:=0; yy:=0;
55     for j:=i+1 to n do
56     begin
57       ans:=ans+(x[j]-x[i])*yy-(y[j]-y[i])*xx;
58       xx:=xx+x[j]-x[i]; yy:=yy+y[j]-y[i];
59     end;
60   end;
61   writeln(abs(ans)/2:0:1);
62 end.