#include <iostream>
#include <stdio.h>
#include <iomanip>
using namespace std;

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

    double x1, x2, y1, y2, xi, yi, ap=0, ai;
    int n;
    cin>>n;

    scanf("%lf %lf \n", &xi, &yi);
    x1=xi;
    y1=yi;

    for(int i=2; i<=n; i++)
    {
        scanf("%lf %lf \n", &x2, &y2);
        ai=((x1*y2)-(x2*y1));
        ap+=ai;
        x1=x2;
        y1=y2;
    }

    ap+=((x2*yi)-(xi*y2));
    ap/=2.0;
    printf("%.6lf", ap);


    return 0;
}