uva-two trees

很久之前的一道水题，代码如下

#include <stdio.h>

int main()
{
double xa,ya,xb,yb;
while(scanf("%lf %lf %lf %lf",&xa,&ya,&xb,&yb)!=EOF)
{
double xo,yo,xc,yc,xd,yd;
if(xa==xb&&ya==yb)
printf("Impossible.\n");
else
{xo=(xa+xb)/2;
yo=(ya+yb)/2;
xc=xo+yo-ya;
yc=yo+xa-xo;
xd=xo+yo-yb;
yd=yo+xb-xo;
printf("%lf %lf %lf %lf\n",xc,yc,xd,yd);
}
}
return 0;
}