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

#define NMAX 100001

int n;
struct point{
    double x,y;
}p[NMAX];

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

void read()
{
    scanf("%d ",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%lf %lf ",&p[i].x,&p[i].y);
    }
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    read();

    point origine;
    origine.x=0;
    origine.y=0;

    double total_area = 0;
    for(int i=0;i<n-1;i++)
        total_area += area(p[i],p[i+1],origine);
    total_area += area(p[0],p[n-1],origine);

    printf("%lf",total_area);

    return 0;
}