poj2954

水题，先用叉积求三角形面积
然后求三边上的点（由公约数上点）a
然后用pick定理S=a+b/2-1
就可以求出内部的点数了

 var x,y,xx,yy,a1,a2,a3,x1,x2,x3,y1,y2,y3:longint;
     s,ans:double;

 function gcd(a,b:longint):longint;
   begin
     if b= then exit(a)
     else exit(gcd(b,a mod b));
   end;

 begin
   readln(x1,y1,x2,y2,x3,y3);
   while not((x1=) and (x2=) and (x3=) and (y1=) and (y2=) and (y3=)) do
   begin
     x:=x2-x1;
     y:=y2-y1;
     xx:=x3-x1;
     yy:=y3-y1;
     s:=abs(x*yy-y*xx)/;
     a1:=gcd(abs(x1-x2),abs(y1-y2));
     a2:=gcd(abs(x1-x3),abs(y1-y3));
     a3:=gcd(abs(x2-x3),abs(y2-y3));
     ans:=s+-(a1+a2+a3)/;
     writeln(ans::);
     readln(x1,y1,x2,y2,x3,y3);
   end;
 end.