#include <bits/stdc++.h>
#define x first
#define y second
using namespace std;

typedef pair<long double, long double> point;

ifstream fin("aria.in");
ofstream fout("aria.out");

point points[100005];
int n;

long double Aria(point pair1, point pair2)
{
    long double  xa = pair1.x;
    long double  ya = pair1.y;
    long double  xb = pair2.x;
    long double  yb = pair2.y;
    return xa*yb - xb*ya;
}

int main()
{
    int i;
    long double aria;
    fin >> n;
    for (i = 1; i <= n; i++)
        fin >> points[i].x >> points[i].y;
    points[n + 1] = points[1];

    aria = 0;
    for (i = 1; i <= n; i++)
        aria += Aria(points[i], points[i + 1]);

    aria *= 0.5;
    fout << fixed << setprecision(6) << aria << "\n";
    return 0;
}