三角形内部坐标为整数值点的个数如何求

#include<bits/stdc++.h>
using namespace std;
int gcd(int x,int y)
{    return y==0?x:gcd(y,x%y);}
int main()
{
    int x1,y1,x2,y2,x3,y3;
    while(scanf("%d%d%d%d%d%d",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF)
    {
        if(!x1&&!y1&&!x2&&!y2&&!x3&&!y3) break;
        double edgnod=1.0*gcd(abs(x1-x2),abs(y1-y2))+gcd(abs(x1-x3),abs(y1-y3))+gcd(abs(x2-x3),abs(y2-y3));
        double s=1.0*abs((x2-x1)*(y3-y1)-(y2-y1)*(x3-x1))/2;
        printf("%.0lf\n",s-edgnod/2+1);
    }
    return 0;

}