#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)
{
    return a.x*b.y - a.y*b.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();

    double total_area = 0;


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

    total_area += area(p[n-1],p[0]);

    printf("%lf",total_area/2);

    return 0;
}