#include <iostream>
#include <stdio.h>
using namespace std;


struct ura {double x, y;};
ura v[100001];

int main() {

    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);

    int n, i;
    double arie;

    scanf ("%d", &n);
    for (i = 1; i <= n; i++)
        scanf ("%lf%lf", &v[i].x, &v[i].y);
    arie = 0;
    for (i = 1; i <= n; i++)
        arie += (v[i].x - v[i % n + 1].x) * (v[i].y + v[i % n + 1].y);
    arie = arie / 2;
    printf ("%lf", arie);

    return 0;
}