bzoj1069

四边形的问题可以转化为三角形处理
穷举对角线，然后处理上下两个三角形，旋转卡壳

var x,y:array[0..5010] of double;
    q:array[0..5010] of longint;
    l,r,i,j,k,t,n:longint;
    ans:double;

function max(a,b:double):double;
  begin
    if a>b then exit(a) else exit(b);
  end;

function cross(i,j,k,r:longint):double;
  begin
    exit((x[i]-x[j])*(y[k]-y[r])-(x[k]-x[r])*(y[i]-y[j]));
  end;

procedure swap(var a,b:double);
  var c:double;
  begin
    c:=a;
    a:=b;
    b:=c;
  end;

procedure sort(l,r: longint);
  var i,j: longint;
      p,q:double;

  begin
    i:=l;
    j:=r;
    p:=x[(l+r) shr 1];
    q:=y[(l+r) shr 1];
    repeat
      while (x[i]<p) or (x[i]=p) and (y[i]<q) do inc(i);
      while (p<x[j]) or (p=x[j]) and (q<y[j]) do dec(j);
      if not(i>j) then
      begin
        swap(x[i],x[j]);
        swap(y[i],y[j]);
        inc(i);
        j:=j-1;
      end;
    until i>j;
    if l<j then sort(l,j);
    if i<r then sort(i,r);
  end;

begin
  readln(n);
  for i:=1 to n do
    readln(x[i],y[i]);
  sort(1,n);
  q[1]:=1;
  t:=1;
  for i:=2 to n do
  begin
    while (t>1) and (cross(q[t],q[t-1],i,q[t-1])<=0) do dec(t);
    inc(t);
    q[t]:=i;
  end;
  k:=t;
  for i:=n-1 downto 1 do
  begin
    while (t>k) and (cross(q[t],q[t-1],i,q[t-1])<=0) do dec(t);
    inc(t);
    q[t]:=i;
  end;
  for i:=2 to t-1 do
    q[i+t-1]:=q[i];
  for i:=1 to t-1 do
  begin
    l:=i+1;
    r:=i+3;
    while (r<i+t-2) and (cross(q[i+2],q[i],q[r],q[i])<cross(q[i+2],q[i],q[r+1],q[i])) do inc(r);
    ans:=max(ans,cross(q[i+1],q[i],q[i+2],q[i])+cross(q[i+2],q[i],q[r],q[i]));
    for j:=i+3 to i+t-2 do
    begin
      while (l<j-1) and (cross(q[l+1],q[i],q[j],q[i])>cross(q[l],q[i],q[j],q[i])) do inc(l);
      while (r<i+t-2) and (cross(q[j],q[i],q[r+1],q[i])>cross(q[j],q[i],q[r],q[i])) do inc(r);
      ans:=max(ans,cross(q[l],q[i],q[j],q[i])+cross(q[j],q[i],q[r],q[i]));
    end;
  end;
  ans:=ans/2;
  writeln(ans:0:3);
end.