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

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

const int MAXN = 100005;
int N;
long double aria;
double x[MAXN], y[MAXN];

int main() {
    fin >> N;
    for (int i = 0; i < N; ++i)
        fin >> x[i] >> y[i];
    x[N] = x[0]; y[N] = y[0];
    for (int i = 0; i < N; ++i)
        aria += x[i] * y[i + 1] - x[i + 1] * y[i];
    fout << fixed << setprecision(6) << abs(0.5 * aria) << '\n';
    fin.close();
    fout.close();
    return 0;
}