#include <fstream>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct Point
{
    double x,y;
};

const int NMax = 100005;
int N;
Point P[NMax];

int main()
{
    double S = 0;
    fin>>N;
    for(int i = 1; i <= N; ++i)
        {
            fin>>P[i].x>>P[i].y;
        }

    P[N+1] = P[1];

    for(int i = 1; i <= N; ++i)
        {
            S += P[i].x*P[i+1].y - P[i+1].x*P[i].y;
        }

    fout << S / 2 <<"\n";

    return 0;
}