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

struct punct
{
    double x, y;
}v[100003];

double aria(punct a, punct b)
{
    return a.x*b.y-b.x*a.y;
}

void citire(int n)
{
    for(int i=1; i<=n; i++)
        scanf("%lf %lf \n", &v[i].x, &v[i].y);
}

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

    int n;
    double ap=0;
    scanf("%d \n", &n);

    citire(n);
    for(int i=1; i<n; i++)
        ap+=aria(v[i],v[i+1]);

    ap+=aria(v[n],v[1]);
    ap/=2.0;
    printf("%.5lf", ap);


    return 0;
}