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

ifstream fin("aria.in");
ofstream fout("aria.out");
const int dim = 100001;
struct Poligon {double x, y;};
int n;
Poligon P[dim];

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

double Arie (Poligon Pol[dim], int n) {
    double arie=0.0;
    for(int i=2; i<=n+1; i++)
        arie+=P[i-1].x*P[i].y-P[i].x-P[i-1].y;
    return arie;
}
int main()
{
    Read();
    fout << Arie(P,n);
    return 0;
}