1 #include <stdio.h>
  2 #include <string.h>
  3 #include <stdlib.h>
  4 #include <iostream>
  5 #include <algorithm>
  6 #include <math.h>
  7
  8 #define MAXX 1005
  9 #define eps 1e-8
 10 using namespace std;
 11
 12 typedef struct
 13 {
 14     double x;
 15     double y;
 16 }point;
 17
 18 typedef struct
 19 {
 20     point st;
 21     point ed;
 22 }line;
 23
 24 point p[MAXX];
 25 line li[MAXX];
 26
 27 double crossProduct(point a,point b,point c)
 28 {
 29     return (c.x-a.x)*(b.y-a.y)-(c.y-a.y)*(b.x-a.x);
 30 }
 31
 32 double dist(point a,point b)
 33 {
 34     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
 35 }
 36
 37 bool  xy(double x,double y){ return x < y - eps; }
 38 bool  dy(double x,double y){ return x > y + eps; }
 39 bool xyd(double x,double y){ return x < y + eps; }
 40 bool dyd(double x,double y){ return x > y - eps; }
 41 bool  dd(double x,double y){ return fabs(x-y)<eps; }
 42
 43 bool onSegment(point a,point b,point c)
 44 {
 45     double maxx=max(a.x,b.x);
 46     double maxy=max(a.y,b.y);
 47     double minx=min(a.x,b.x);
 48     double miny=min(a.y,b.y);
 49
 50     if(dd(crossProduct(a,b,c),0.0)&&xyd(c.x,maxx)&&dyd(c.x,minx)
 51        &&xyd(c.y,maxy)&&dyd(c.y,miny))
 52         return true;
 53     return false;
 54 }
 55
 56 bool segIntersect(point p1,point p2,point p3,point p4)
 57 {
 58     double d1=crossProduct(p3,p4,p1);
 59     double d2=crossProduct(p3,p4,p2);
 60     double d3=crossProduct(p1,p2,p3);
 61     double d4=crossProduct(p1,p2,p4);
 62
 63     if(xy(d1*d2,0.0)&&xy(d3*d4,0.0))
 64         return true;
 65     if(dd(d1,0.0)&&onSegment(p3,p4,p1))
 66         return true;
 67     if(dd(d2,0.0)&&onSegment(p3,p4,p2))
 68         return true;
 69     if(dd(d3,0.0)&&onSegment(p1,p2,p3))
 70         return true;
 71     if(dd(d4,0.0)&&onSegment(p1,p2,p4))
 72         return true;
 73     return false;
 74 }
 75
 76 double Area(int n)
 77 {
 78     double ans=0.0;
 79
 80     for(int i=2; i<n; i++)
 81     {
 82         ans+=crossProduct(p[0],p[i-1],p[i]);
 83     }
 84     return fabs(ans)/2.0;
 85 }
 86
 87 int main()
 88 {
 89     int n,m,i,j;
 90     double x,y;
 91     int cas=1;
 92     while(scanf("%d",&n)!=EOF&&n)
 93     {
 94         for(i=0; i<n; i++)
 95         {
 96             scanf("%lf%lf",&p[i].x,&p[i].y);
 97         }
 98
 99         for(i=0; i<n-1; i++)
100         {
101             li[i].st.x=p[i].x;
102             li[i].st.y=p[i].y;
103             li[i].ed.x=p[i+1].x;
104             li[i].ed.y=p[i+1].y;
105         }
106         li[n-1].st.x=p[n-1].x;
107         li[n-1].st.y=p[n-1].y;
108         li[n-1].ed.x=p[0].x;
109         li[n-1].ed.y=p[0].y;
110         bool flag=false;
111         for(i=0; i<n; i++)
112         {
113             for(j=i+2; j<n; j++)
114             {
115                     if(i == 0 && j == n-1)continue;
116                     /*if((li[i].st.x == li[j].st.x && li[i].st.y == li[j].st.y)
117                        || li[i].st.x == li[j].ed.x && li[i].st.y == li[j].ed.y
118                        || li[i].ed.x == li[j].st.x && li[i].ed.y == li[j].st.y
119                        || li[i].ed.x == li[j].ed.x && li[i].ed.y == li[j].ed.y)
120                         continue;*/
121                     if(segIntersect(li[i].st,li[i].ed,li[j].st,li[j].ed))
122                     {
123                         flag=true;
124                         break;
125                     }
126             }
127         }
128
129         if(flag || n<3)
130         {
131             printf("Figure %d: Impossible\n",cas++);
132         }
133         else
134         {
135             double ans=Area(n);
136             printf("Figure %d: %.2lf\n",cas++,ans);
137         }
138         printf("\n");
139     }
140     return 0;
141 }