1 #include <stdio.h> 2 #include <math.h> 3  4 #define MISS 0.0000001 5  6 int det(double x1,double y1,double x2,double y2,double x3,double y3){ 7     double w1,w2; 8     w1=x1*y2+x2*y3+x3*y1; 9     w2=x2*y1+x3*y2+x1*y3;10     if(fabs(w1-w2)<MISS)11         return 0;12     else if(w1>w2)13             return 1;14         else if(w1<w2)15                 return -1;16 }17 18 int main(){19     int n,i,j,count=0,temp;20     double x[1000],y[1000],area;21     while(scanf("%d",&n)&&n){22         count++;23         if(count>1)24             printf("\n");25         printf("Figure %d: ",count);26         for(i=0;i<n;i++)27             scanf("%lf%lf",&x[i],&y[i]);28         if(n<3){29             printf("Impossible\n");30             continue;31         }32         for(i=0;i<n;i++){33             for(j=(i+1)/n;j<i-1;j++){34                 temp=(i+1)%n;35                 if (det(x[j],y[j],x[i],y[i],x[j+1],y[j+1])*det(x[j],y[j],x[temp],y[temp],x[j+1],y[j+1])<=0&&36                 det(x[i],y[i],x[j],y[j],x[temp],y[temp])*det(x[i],y[i],x[j+1],y[j+1],x[temp],y[temp])<=0){37                     printf("Impossible\n");38                     goto quit;39                 }40             }41         }42         area=0;43         for(i=0;i<n;i++)44             area+=x[i]*y[(i+1)%n]-y[i]*x[(i+1)%n];45         printf("%.2lf\n",fabs(area)/2);46         quit:;47     }48     return 0;49 }