#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int n;
struct punct{
    double x;
    double y;
}a[100001];

void Citire()
{
    freopen("aria.in", "r", stdin);

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

void Aria()
{
    double aria=0;
    for(int j=0; j<=n; j++)
        aria+=a[j].x*a[j+1].y-a[j].y*a[j+1].x;
    printf ("%lf", aria/2);
}

int main()
{
    freopen("aria.out", "w", stdout);

    Citire();
    Aria();
    return 0;
}