#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

const int Nmax = 1e5+5;
long double aria, x[Nmax], y[Nmax];
int N;

int main() {
    fin >> N;
    for(int i = 1; i <= N; i++)
        fin >> x[i] >> y[i];
    x[N + 1] = x[1];
    y[N + 1] = y[1];
    for(int i = 1; i <= N; i++)
        aria += x[i] * y[i + 1] - x[i + 1] * y[i];
    fout << setprecision(5) << fixed << aria * 0.5;
}