#include<stdio.h>
#include<math.h>
int main(){
  int n,i,gx,gy,bx,by,x,y;
  double s=0;
  scanf("%d",&n);
  scanf("%d %d %d %d",&gx,&gy,&bx,&by);
  bx-=gx;
  by-=gy;
  n-=2;
  while(n--){
    scanf("%d %d",&x,&y);
    x-=gx;
    y-=gy;
    s+=abs(bx*y-by*x)/2.0;
    bx=x;
    by=y;
  }
  printf("%f\n",s);
  return 0;
}