#include <bits/stdc++.h>
using namespace std;

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

const int dim=100009;

struct punct{
long double x,y;}v[dim];

int n;
long double arie;

long double determinat(punct A,punct B,punct C){
    return A.x*B.y+B.x*C.y+C.x*A.y-B.y*C.x-C.y*A.x-A.y*B.x;
}

signed main(){

        fin>>n;
    for(int i=1;i<=n;i++){
        fin>>v[i].x>>v[i].y;
    }
    v[n+1]=v[1];
    punct O={0,0};
    for(int i=1;i<=n;i++)
        arie+=determinat(O,v[i],v[i+1]);
    fout<<fixed<<setprecision(5)<<max(arie,-arie)/2;
}