#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;

int n;
struct pct
{
    double x,y;
};

int main()
{
    int i;
    double sol=0,part;
    pct prec,act,prim;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf%lf",&prec.x,&prec.y);
    prim.x=prec.x; prim.y=prec.y;
    scanf("%lf%lf",&act.x,&act.y);
    part=prec.x*act.y-prec.y*act.x;
    part/=2;
    sol+=part;
    for(i=3;i<=n;i++)
    {
        prec.x=act.x; prec.y=act.y;
        scanf("%lf%lf",&act.x,&act.y);
        part=prec.x*act.y-prec.y*act.x;
        part/=2;
        sol+=part;
    }
    scanf("%lf%lf",&act.x,&act.y);
    part=prim.y*act.x-prim.x*act.y;
    part/=2;
    sol+=part;
    printf("%lf\n",fabs(sol));
    fclose(stdin);
    fclose(stdout);
    return 0;
}