#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

struct elem {double x;double y;}Punct[100010];
int i,n;
double Aria;

void citire() {

    ifstream in("aria.in");
    in>>n;
    for(i=1;i<=n;i++)
        in>>Punct[i].x>>Punct[i].y;
    in.close();

}

void solve() {

    Punct[n+1].x=Punct[1].x;
    Punct[n+1].y=Punct[1].y;
    for(i=1;i<=n;i++)
        Aria+=Punct[i].x*Punct[i+1].y-Punct[i].y*Punct[i+1].x;
    Aria=Aria/2;
    if(Aria<0)
        Aria=-Aria;

}

void afisare() {

    ofstream out("aria.out");
    out<<fixed<<setprecision(10)<<Aria<<'\n';
    out.close();

}

int main()
{
    citire();
    solve();
    afisare();
    return 0;
}