#include <fstream>
#include <iomanip>
#include <vector>

#define in "aria.in"
#define out "aria.out"
typedef std :: pair < double, double > PAIR;
typedef std :: vector < PAIR > Array;

std :: ifstream f(in);
std :: ofstream g(out);

int N;
Array V;

int main() {
    f >> N;
    V.resize(N + 5);
    for(int i = 0; i < N; ++i)  f >> V[i].first >> V[i].second;
    V[N] = V[0];

    double Aria = 0;
    for(int i = 0; i < N; ++i)
        Aria += (V[i].first * V[i + 1].second - V[i].second * V[i + 1].first);

    if(Aria < 0)    Aria = -Aria;
    Aria *= 0.5;

    g << std :: fixed << std :: setprecision(7) << Aria << '\n';

    g.close();
    return 0;
}