#include <bits/stdc++.h>

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

int N,i;
double x[100005], y[100005];

int main() {
    fin >> N;
    for (i = 0; i < N; ++i) {
        fin >> x[i] >> y[i];
    }

    double area = 0.0;
    for (i = 0; i < N; ++i) {
        int next = (i + 1) % N;
        area += (x[i] * y[next] - x[next] * y[i]);
    }

    if (area < 0) area = -area;
    area /= 2.0;

    fout << fixed << setprecision(5) << area << endl;

    return 0;
}