#include<cstdio>
using namespace std;

int main()
{
    double x0,y0,x1,y1,x2,y2,s=0;
    int i,n;
    FILE *f=fopen("aria.in","r");
    fscanf(f,"%d",&n);
    fscanf(f,"%lf%lf",&x0,&y0);
    x1=x0;y1=y0;
    for(i=2;i<=n;i++)
    {
        fscanf(f,"%lf%lf",&x2,&y2);
        s+=0.5*(x1*y2-y1*x2);
        x1=x2;y1=y2;
    }
    s+=0.5*(x1*y0-y1*x0);
    fclose(f);
    f=fopen("aria.out","w");
    fprintf(f,"%lf",s);
    fclose(f);
    return 0;
}