#include <bits/stdc++.h>

using namespace std;

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

struct vec
{
    double x, y;
}v[100001];

int main()
{
    int n;
    double s = 0;
    in >> n;
    for (int i = 1; i <= n; ++i)
        in >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    for (int i = 1; i <= n; ++i)
        s += v[i + 1].y * v[i].x - v[i + 1].x * v[i].y;
    return out << setprecision(6) << fixed << fabs(s / 2.0), 0;
    return 0;
}