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

int n;
double arie;

struct point {
    int x,y;
} v[100001];

double area(point a, point b, point c) {
    a.x -= c.x;
    b.x -= c.x;
    a.y -= c.y;
    b.y -= c.y;

    return fabs((double)(a.x*b.y-a.y*b.x)/2);
}

int main() {

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

    fin >> n;

    for (int i = 1; i<=n; i++) {
        fin >> v[i].x >> v[i].y;
    }

    for (int i = 2; i<n; i++) {
        arie += area(v[1], v[i], v[i+1]);
    }

    fout << arie;


    fin.close();
    fout.close();

    return 0;
}