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

struct point{
    long double x, y;
};
int n;
long double modul(long double x){
    return x>0.0 ? x : -x;
}
vector<point> points;
int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    cin>>n;
    for (int i=0; i<n; ++i){
        points.push_back({0, 0});
        cin>>points.back().x>>points.back().y;
    }
    points.push_back(points[0]);
    long double aria=0.0;
    for (int i=0; i<points.size()-1; ++i){
        aria+=points[i].x*points[i+1].y-points[i+1].x*points[i].y;
    }
    cout<<fixed<<modul(aria)/2.0;
    return 0;
}