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

#define in "aria.in"
#define out "aria.out"
#define Max_Size 100009
typedef std :: pair < double, double > PAIR;
typedef std :: vector < PAIR > Array;

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

int N;
PAIR V[Max_Size];

int main() {
    f >> N;
    for(int i = 1; i <= N; ++i) f >> V[i].first >> V[i].second;
    V[N + 1] = V[1];

    double Aria = 0.0;
    for(int i = 1; i <= N; ++i)
        Aria = (Aria + V[i].first * V[i + 1].second - V[i + 1].first * V[i].second);
    if(Aria < 0) Aria = -Aria;
    Aria /= 2.0;

    g << std :: fixed << std :: setprecision(5) << Aria << '\n';

    g.close();
    return 0;
}