#include<fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct {
    double x; double y;
};
punct p[100002];
int n, i;
double s;

double aria(punct p1, punct p2){
    return (p1.x * p2.y - p2.x * p1.y) / 2;
}

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