#include <cstdio>
#include <iostream>
#define x first
#define y second

using namespace std;

const int nmx = 100002;

int n;
pair <double,double> v[nmx];

double determinant(pair <double,double> p1, pair <double,double> p2, pair <double,double> p3)
{
    return (p2.x - p1.x) * (p3.y - p1.y) - (p3.x - p1.x) * (p2.y - p1.y);
}

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

    scanf("%d", &n);
    for(int i = 1; i <= n; ++i)
        scanf("%lf %lf", &v[i].x, &v[i].y);

    double aria = 0;

    for(int i = 1; i < n; ++i)
        aria += determinant(make_pair(0,0),v[i],v[i+1]);
    aria += determinant(make_pair(0,0),v[n],v[1]);

    aria /= 2;

    printf("%lf", aria > 0 ? aria : - aria);

    return 0;
}