#include <fstream>
#include <iomanip>
using namespace std;
ofstream fout("aria.out");
ifstream fin("aria.in");

const int NMAX = 100005;

int N;
double Aria;

struct punct { double x, y; } dicky[NMAX];

int main()
{
    fin >> N;
    for(int i=1; i<=N; i++) fin >> dicky[i].x >> dicky[i].y;

    for(int i=1; i<=N; i++) Aria += dicky[i].x * dicky[i+1].y - dicky[i+1].x * dicky[i].y;
    Aria += dicky[N].x * dicky[1].y - dicky[1].x * dicky[N].y;

    if(Aria < 0.0)
        Aria = -Aria;
    Aria /= 2;

    fout << setprecision(6) << Aria << '\n';
    return 0;
}