#include<fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct {
    double x; double y;
};
punct v[100001];
int s, n, i;

int aria (punct p1, punct p2){
    return (p1.x * p2.y - p2.x * p1.y);
}

int main(){
    fin >> n;
    for (i = 1; i <= n; i++)
        fin >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    for (i = 1; i <= n; i++)
        s = s + aria(v[i], v[i + 1]);
    fout << s / 2 << '\n';
    fin.close();
    fout.close();
}