#include <fstream>
#include <iomanip>
#include <cmath>
#define NM 100010
#define x first
#define y second

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int N;
pair<double, double> V[NM];
double ANS;
int i;


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

    V[N+1]=V[1]; V[0]=V[N];

    for (i=1; i<=N; i++)
        ANS+=V[i].x*(V[i+1].y-V[i-1].y);

    g << fixed << setprecision(6) << fabs(ANS*0.5) << '\n';

    f.close();
    g.close();

    return 0;
}