#include <fstream>

#include <cmath>
using namespace std;

const int MAX_N = 100002;

int N;
double A;
double v[MAX_N][2];

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

    f >> N;
    for(int i = 1; i <= N; ++i)
        f >> v[i][0] >> v[i][1];
    v[N+1][0] = v[1][0], v[N+1][1] = v[1][1];

    for(int i = 1; i <= N; ++i)
        A += (long long) (v[i][0] * v[i+1][1]) - (long long) (v[i+1][0] * v[i][1]);
    A = fabs(A/2);

    g << A << '\n';

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

    return 0;
}