#include<cstdio>
#include<cmath>
int n,i;
struct P 
{
      double x,y;
}v[100005];
double s;
int main() 
{
      freopen("aria.in","r",stdin),
      freopen("aria.out","w",stdout),
      scanf("%d",&n);
      for(i=0;i<n;++i) 
             scanf("%lf%lf",&v[i].x,&v[i].y);
      for(v[n]=v[0],i=0;i<n;++i)
             s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
      printf("%.5lf",fabs(s/2.0));
      return 0;
}