#include <iostream>
#include <fstream>

using namespace std;

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

struct coord
{
    double x, y;
} v[100001];

int N, i;
long S;

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++)
        S += (v[i].x*v[i+1].y - v[i+1].x*v[i].y);
    fout << S/2;
    fin.close();
    return 0;
}