#include<cstdio>
using namespace std;
int n,i;
double s;
struct str{double x; double y;} ;
str v[100100];
FILE *fin=fopen("aria.in","r");
FILE *fout=fopen("aria.out","w");
int main ()
{
    fscanf(fin,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(fin,"%lf %lf",&v[i].x,&v[i].y);
    s=s+v[n].x*v[1].y-v[1].x*v[n].y;
    for(i=2;i<=n;i++)
        s=s+v[i-1].x*v[i].y-v[i].x*v[i-1].y;
    s=s/(2*1.0);
    fprintf(fout,"%lf",s);
return 0;
}