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

using namespace std;

const int MAXN = 100010;
long double x[MAXN], y[MAXN], solution = 0;
int n;

int main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin >> n;
    for (int i = 0; i < n; i++)
        fin >> x[i] >> y[i];
    for (int i = 0; i < n; i++)
        solution += ((y[i] + y[(i + 1) % n]) * (x[(i + 1) % n] - x[i]));
    solution = abs(solution / 2);
    fout << fixed;
    fout << setprecision(5) << solution;
    return 0;
}