#include <fstream>
#include <cmath>
#define NMAX 100000 + 5
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

struct aria {
    long double x, y;
} v[NMAX];

int N;
long double sum;

int main()
{
    fin >> N;
    for(int i = 0; i < N; ++ i)
        fin >> v[i].x >> v[i].y;
    v[N] = v[0];

    for(int i = 0; i < N; ++ i)
        sum += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);

    fout << fabs(sum / 2.0);
    return 0;
}