#include<fstream>
#include<iomanip>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

const int Nmax=100003;

struct point
{
    double x,y;
};
point A[Nmax];
int n,i;
double Aria;

int main()
{
    cin >> n;
    for (i=1;i<=n;i++) cin >> A[i].x >> A[i].y;
    A[n+1]=A[1];

    for (i=1;i<=n;i++) Aria += A[i].x*A[i+1].y - A[i].y*A[i+1].x;
    Aria /= 2;
    if (Aria < 0) Aria=-Aria;
    cout << fixed << setprecision(10) << Aria;
    return 0;
}