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

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

const int MAXN = 100005;

int N;
double aria;
double x[MAXN], y[MAXN];

int main() {
    fin >> N;
    for (int i = 1; i <= N + 1; ++i) {
        fin >> x[i] >> y[i];
        x[N + 1] = x[1]; y[N + 1] = y[1];
        aria += (x[i - 1] * y[i] - x[i] * y[i - 1]);
    }
    fout << fixed << setprecision(9) << aria * 0.5;
    fin.close();
    fout.close();
    return 0;
}