#include <cstdio>
#define nmax 100002
using namespace std;
FILE *f1=fopen("aria.in","r"),*f2=fopen("aria.out","w");
int n,i;
double A;
struct point
{
   double x,y;
}v[nmax];
int main()
{

    fscanf(f1,"%d",&n);
    for(i=1;i<=n;i++)
    fscanf(f1,"%lf%lf",&v[i].x,&v[i].y);
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        A+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fprintf(f2,"%lf",A/2.0);
    fclose(f1);
    fclose(f2);
    return 0;
}