#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
const int MAXN = 100010;

struct Point
{
    int x, y;
}pts[MAXN];

int n;

void read()
{
    fin >> n;
    for (int i = 0; i < n; ++i)
        fin >> pts[i].x >> pts[i].y;
}

void solve()
{
    double area{0};
    for (int i = 0; i < n; ++i)
        area += (pts[i].x * pts[(i + 1) % n].y  - pts[(i + 1) % n].x * pts[i].y);
    area = abs(area / 2);
    fout << setprecision(5) << fixed << area << '\n';
}

int main()
{
    read();
    solve();
    return 0;
}