#include<iostream>
#include<fstream>
using namespace std;

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

struct punct {
    double x, y;
};

int n, i;
double a;
punct v[100001];

int main() {
    fin >> n;
    for(i = 0; i < n; i++) {
        fin >> v[i].x >> v[i].y;
    }
    v[n] = v[0];
    for(i = 0; i < n; i++) {
        a += (v[i].x*v[i+1].y - v[i].y*v[i+1].x);
    }
    fout << a / 2;
    fin.close();
    fout.close();
    return 0;
}