#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
double x[100005], y[100005], sol;
int n, i;
int main() {
    f >> n;
    for (i = 1; i <= n; i++)
        f >> x[i] >> y[i];
    x[n+1] = x[1], y[n+1] = y[1];

    for (i = 1; i <= n; i++)
        sol += x[i]*y[i+1] - x[i+1]*y[i];
    if (sol < 0) sol = -sol;
    sol/=2;
    g << fixed << setprecision(5) << sol;

    return 0;
}