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

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

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

double Arie_Triunghi (Punct A, Punct B, Punct C) {
    return ((B.x-A.x)*(C.y-A.y)-(C.x-A.x)*(B.y-A.y))/2;
}
double Arie (Punct Pol[dim], int n) {
    double arie=0;
    for(int i=2; i<n; i++)
        arie+=Arie_Triunghi(P[1],P[i],P[i+1]);
    return arie;
}
int main()
{
    Read();

    fout << Arie(P,n);
    return 0;
}