#include <cstdio>
#define N 100002

using namespace std;
FILE* f, * g;

struct bla
{
    double x, y;
}v[N];
double  det(bla A, bla B, bla C)
{
    double x1 = A.x, x2 = B.x, x3 = C.x, y1 = A.y, y2 = B.y, y3 = C.y;
    return (x1 * y2 + x2 * y3 + x3 * y1 - x3 * y2 - y3 * x1 - x2 * y1);
}
int main()
{
    f = fopen("aria.in", "r");
    g = fopen("aria.out", "w");
    int n;
    fscanf(f, "%d", &n);
    for (int i = 1;i <= n;++i)
        fscanf(f, "%lf %lf", &v[i].x, &v[i].y);
    double A = 0;
    for (int i = 2;i < n;++i)
        A = A + det(v[1], v[i], v[i + 1]);
    A = A * 0.5;
    fprintf(g, "%f", A);

    fclose(f);
    fclose(g);
    return 0;
}