// https://www.infoarena.ro/problema/aria
#include <iostream>
#include <vector>
#include <iomanip>
using namespace std;

struct point{
    long double x, y;
};
vector<point> p;
int n;
int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    cin>>n;
    for (int i=0; i<n; ++i){
        p.push_back({0,0});
        cin>>p.back().x>>p.back().y;
    }
    p.push_back(p[0]);
    long double area=.0;
    for (int i=0; i<p.size()-1; ++i){
        area+=p[i].x*p[i+1].y-p[i].y*p[i+1].x;
    }
    area*=.5;
    cout<<fixed<<setprecision(5)<<area;
}