#include<bits/stdc++.h>
#define EPS 1e-10
#define equals(a,b) (fabs((a)-(b)) < EPS)
#define rep(i,n) for(int i=0;i<n;++i)
typedef long long ll;
using namespace std;

static const int COUNTER_CLOCKWISE = 1;
static const int CLOCKWISE = -1;
static const int ONLINE_BACK = 2;
static const int ONLINE_FRONT = -2;
static const int ON_SEGMENT = 0;

// 点
struct Point {
    double x,y;
    Point(){}
    Point(double x, double y) : x(x),y(y){}
    Point operator+(Point p) {return Point(x+p.x, y+p.y);}
    Point operator-(Point p) {return Point(x-p.x, y-p.y);}
    Point operator*(double k){return Point(x*k,y*k);}
    Point operator/(double k){return Point(x/k,y/k);}
    double norm(){return x*x+y*y;}
    double abs(){sqrt(norm());}
    bool operator == (const Point &p) const {return equals(x,p.x)&&equals(y,p.y);}
};

typedef Point P;
typedef vector<Point> Polygon;

double cross(Point a, Point b) {return a.x*b.y-a.y*b.x;}

double area(Polygon s){
    double res=0;
    for(int i=0;i<(int)s.size();i++){
        res+=cross(s[i],s[(i+1)%s.size()])/2.0;
    }
    return res;
}

int main() {
    int n; cin>>n;
    Polygon pl;
    rep(i,n) {
        double x,y; cin>>x>>y;
        pl.push_back(Point(x,y));
    }
    double ans = area(pl);
    cout << fixed << setprecision(1);
    cout << ans << endl;
}
