#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

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

struct Point {
    int x, y;
};

int n;
long long s;
Point v[100005];

long long get_signed_area(Point a, Point b) {
    return (long long)a.x * b.y - (long long)b.x * a.y;
}

int main()
{
    in >> n;
    for (int i = 1, x, y; i <= n; ++i) {
        in >> x >> y;
        v[i] = {x, y};
    }

    v[n + 1] = v[1];

    for (int i = 1; i <= n; ++i) {
        s += get_signed_area(v[i], v[i + 1]);
    }

    long double arie = (long double)s / 2;
    out << setprecision(5) << arie;

    return 0;
}