#include <bits/stdc++.h>

using namespace std;

struct Coord
{
    double x, y;
} v[100005];

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    scanf("%d", &n);
    for(int i = 0; i < n; i++)
        scanf("%lf%lf", &v[i].x, &v[i].y);
    double rez = 0;
    v[n] = v[0];
    for(int i = 0; i < n; i++)
        rez += v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
    rez /= 2.;
    printf("%f", rez);
    return 0;
}