#include <bits/stdc++.h>

#define Nmax 100005
#define eps 1e-8

using namespace std;

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

struct point
{
    double x, y;
};
point P[Nmax];
int N;
int i;
double area;

int main()
{
    fin >> N;
    for(i = 1; i <= N; i++)
        fin >> P[i].x >> P[i].y;
    if(N < 3)
    {
        fout << "0\n";
        return 0;
    }
    P[N + 1] = P[1];
    for(i = 1; i <= N; i++)
        area += (P[i].x * P[i + 1].y - P[i + 1].x * P[i].y);
    area /= 2.;
    fout << fixed << setprecision(8) << area << "\n";
    return 0;
}