#include<cstdio>
using namespace std;

int main()
{
    double fx,fy,x1,y1,x2,y2;
    long double s=0;
    int i,n;
    FILE *f=fopen("aria.in","r");
    fscanf(f,"%d\n%lf %lf\n",&n,&fx,&fy);
    x1=fx;y1=fy;
    for(i=2;i<=n;i++)
    {
        fscanf(f,"%lf %lf\n",&x2,&y2);
        s+=x1*y2-x2*y1;
        x1=x2;y1=y2;
    }
    s+=x2*fy-fx*y2;
    f=fopen("aria.out","w");
    fprintf(f,"%lf",(double)(s/2));
    fclose(f);
    return 0;
}