#include <iostream>
#include <fstream>
#include <iomanip>
#define Dmax 100010
using namespace std;

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

struct COORDONATE {double x, y;}v[Dmax]; double SUMA; int N;

int main()
{
    in >> N;

    for(int i=1; i<=N; i++) in >> v[i].x >> v[i].y;

    v[N+1].x=v[1].x; v[N+1].y=v[1].y; //CONSIDERAM PUNCTUL A(i)=A(N+1)

    for(int i=1; i<=N; i++)
        SUMA=SUMA+((v[i].x*v[i+1].y)-(v[i+1].x*v[i].y));

    SUMA=SUMA/2;

    out << fixed << setprecision(6) << SUMA;

    return 0;
}