#include <fstream>
#include <iomanip>
#include <cmath>
#define nmax 100003
using namespace std;
ifstream is ("aria.in");
ofstream os ("aria.out");

struct Point{
    double x, y;
} P[nmax];
int N;
double A;

void Read();
inline double Aria(Point& A, Point& B){
    return (A.x * B.y - A.y * B.x);
}

int main()
{
    Read();

    P[0] = P[N];
    for(int i = 1; i <= N; ++i)
        A += Aria(P[i], P[i-1]);

    os << fixed << setprecision(6) << fabs(A/2.0);

    is.close();
    os.close();
    return 0;
}
void Read() {
    is >> N;
    for(int i = 1; i <= N; ++i)
        is >> P[i].x >> P[i].y;
}