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

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n, i;
float s;
int x[100001], y[100001];

int main()
{
    f >> n;
    for (i = 1; i <= n; i++)
        f >> x[i] >> y[i];
    for (i = 1; i < n; i++)
        s += x[i]*y[i+1] - x[i+1]*y[i];
    s += x[n]*y[1] - x[1]*y[n];
    g << setprecision(5) << fabs(s/2);
    return 0;
}