#include <fstream>
#define N 100010

using namespace std;

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

void scan()
{
    ifstream in("aria.in");
    in >> n;
    for (int i = 0; i < n; i++)
        in >> x[i] >> y[i];
    in.close();
}

double solve()
{
    x[n] = x[0];
    y[n] = y[0];
    double rez = 0;
    for (int i = 0; i < n; i++)
        rez += x[i] * y[i + 1] - x[i + 1] * y[i];
    rez /= 2;
    return rez >= 0 ? rez : -rez;
}

void print(double aria)
{
    ofstream out("aria.out");
    out << aria << endl;
    out.close();
}

int main()
{
    scan();
    double aria = solve();
    print(aria);
}