#include <cstdio>
using namespace std;
FILE *fi,*fo;
struct Punct
{
    double x;
    double y;
};
Punct a,b,pr;
double A;
int i,n;
int main()
{
    fi=fopen("aria.in","r");
    fo=fopen("aria.out","w");
    fscanf(fi,"%d%lf%lf",&n,&a.x,&a.y);
    pr=a;
    for(i=2;i<=n;i++)
    {
        fscanf(fi,"%lf%lf",&b.x,&b.y);
        A+=(a.x*b.y-b.x*a.y)/2;
        a=b;
    }
    b=pr;
    A+=(a.x*b.y-b.x*a.y)/2;
    fprintf(fo,"%lf\n",A);
    fclose(fi);
    fclose(fo);
    return 0;
}