#include <bits/stdc++.h>
#define NMax 100007

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct DB
{
    double x, y;
};

DB a[NMax];
long
double aria = 0.0;

int n;
void Citire()
{
    int i;

    fin >> n;

    for(i = 0; i < n; i++)
        fin >> a[i].x >> a[i].y;
}

void Rezolva()
{
    int i;

    a[n] = a[0];
    for (i = 0; i < n; i++)
        aria += (a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);

    fout << setprecision(7) << fixed << aria / 2.0;
}
int main()
{
    Citire();
    Rezolva();
    return 0;
}