1010 Area

 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 }