#include <fstream>
#include <iomanip>
using namespace std;

ifstream is ("aria.in");
ofstream os ("aria.out");

int N;

#define PDD pair<double,double>
#define x first
#define y second

PDD P[100001];
double sum;

double CrossProduct(PDD A, PDD B);

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

    for (int i = 1; i < N; ++i)
        sum += CrossProduct(P[i], P[i+1]);
    sum += CrossProduct(P[N], P[1]);
    sum /= 2;
    os << fixed;
    os << setprecision(5) << sum;
}

double CrossProduct(PDD A, PDD B)
{
    return (A.x * B.y) - (A.y * B.x);
};