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108 lines (92 loc) · 1.62 KB
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/*
zju 1010 Area
00:00.20 480k
2004.07.29 by adai
*/
#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
#define MAXN 1000
class pos{
public:
double x;
double y;
pos &operator = (const pos &cc){
x=cc.x;
y=cc.y;
return *this;
}
};
class line{
public:
pos a;
pos b;
};
double multi(pos p1,pos p2,pos p0){
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
double max(double a,double b){
return a>b?a:b;
}
double min(double a,double b){
return a<b?a:b;
}
bool across(line u,line v){
if(
(max(u.a.x,u.b.x)>=min(v.a.x,v.b.x))&&
(max(v.a.x,v.b.x)>=min(u.a.x,u.b.x))&&
(max(u.a.y,u.b.y)>=min(v.a.y,v.b.y))&&
(max(v.a.y,v.b.y)>=min(u.a.y,u.b.y))&&
(multi(v.a,u.b,u.a)*multi(u.b,v.b,u.a)>=0)&&
(multi(u.a,v.b,v.a)*multi(v.b,u.b,v.a)>=0)
) return 1;
else return 0;
}
double getarea(pos p[MAXN],int n){
double s1=0,s2=0;
int i;
for (i=0;i<n;i++)
s1+=p[(i+1)%n].y*p[i].x,s2+=p[(i+1)%n].y*p[(i+2)%n].x;
return fabs(s1-s2)/2;
}
pos p[MAXN];
line l[MAXN];
int n;
int main(){
int kase=0;
while(1){
scanf("%d",&n);
if(n==0) break;
for(int i=0;i<n;i++)
cin>>p[i].x>>p[i].y;
for(int i=0;i<n;i++){
l[i].a=p[i];
l[i].b=p[(i+1)%n];
}
if(kase++) printf("\n");
printf("Figure %d: ",kase);
if(n<=2){
printf("Impossible\n");
continue;
}
int isacross=0;
for(int i=0;i<n-2;i++){
for(int j=i+2;j<n;j++){
if((j+1)%n==i) continue;
if(across(l[i],l[j])){
isacross=1;
break;
}
}
if(isacross) break;
}
if(isacross){
printf("Impossible\n");
continue;
}
double area=getarea(p,n);
printf("%.2lf\n",area);
}
return 0;
}