#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

const int NMAX = 100005;
struct Point{
    double x, y;
}v[NMAX];
int n;
double ans;

int main(){
    int i;
    f >> n;
    for(i = 1 ; i <= n ; i++)
        f >> v[i].x >> v[i].y;
    v[0] = v[n];
    v[n + 1] = v[1];

    for(i = 1 ; i <= n ; i++)
        ans += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);

    g << fixed << setprecision(6) << ans / 2.0;

    return 0;
}