#include <iostream>
#include <stdio.h>

using namespace std;

struct point{
    double x,y;
};
point points[100000];

double triangle_area(point p1,point p2)
{
    return (p1.x*p2.y-p2.x*p1.y);
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n;
    scanf("%d",&n);

    for(int i=0;i<n;i++)
        scanf("%lf%lf",&points[i].x,&points[i].y);

    double a=0;
    for(int i=0;i<n-1;i++)
        a+=triangle_area(points[i],points[i+1]);
    a+=triangle_area(points[n-1],points[0]);
    printf("%lf",a/2);
}