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

#define NMAX 100001

int n;

struct point{
    double x,y;
};

double area(point a, point b)
{
    return a.x*b.y - a.y*b.x;
}

double total_area;

void read()
{
    scanf("%d ",&n);
    point p0,p01,p;
    scanf("%lf %lf ",&p0.x,&p0.y);
    p01=p0;
    for(int i=1;i<n;i++)
    {
        scanf("%lf %lf ",&p.x,&p.y);
        total_area += area(p,p0);
        p0 = p;
    }
    total_area += area(p01,p);
    total_area /= 2;
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    read();
    printf("%lf",total_area >=0 ? total_area : -total_area);

    return 0;
}