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

using namespace std;
long double x[100005];
long double y[100005];


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

    int n;
    long double aria = 0;

    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] - y[i]* x[i + 1]);
    }

    if (aria < 0) {
        aria *= -1;
    }

    fout << setprecision(5) << aria / 2;

    return 0;
}