#include <stdio.h> #include <math.h> int main() { double x1,x2,x3,y1,y2,y3,s; int n; while(scanf("%d",&n)!=EOF&&n) { s=0; scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2); for(int i=0;i<n⑵;i++) { scanf(" %lf %lf",&x3,&y3); s=s+((x2-x1)*(y3-y1)-(x3-x1)*(y2-y1))/2; x2=x3,y2=y3; } printf("%.1lf ",s); } return 0; }