#include <iostream>
#include <fstream>
#include <string>
#include <iomanip>
#include <math.h>
#include <stdbool.h>
#include <algorithm>
using namespace std;

class Point {
    public:
    double x, y;

    Point(double x = 0, double y = 0): 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 a) { return Point(a*x, a*y); }
    Point operator / (double a) { return Point(x/a, y/a); }

    double abs() {return sqrt(norm()); }
    double norm() {return x*x + y*y; }
};

typedef Point Vector;


double dot(Vector a, Vector b) {
    return a.x * b.x + a.y * b.y;
}

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

int main(){
    #if 0
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
    #endif

    int n;
    cin >> n;

    Point p0, p1, p2;
    double area = 0;

    for(int i=0; i<n; i++){
        if(i==0)
        {
            cin >> p0.x >> p0.y;
            p1.x = p0.x;
            p1.y = p0.y;
        }
        else
        {
            p1.x = p2.x;
            p1.y = p2.y;
        }
        if(i==n-1){
            p2.x = p0.x;
            p2.y = p0.y;
        }
        else{
            cin >> p2.x >> p2.y;
        }

        area += cross(p1,p2);
    }
    area = fabs(area)/2.0;

    std::cout << std::fixed;
    cout << setprecision(1) << area <<"\n";
    return 0;
}

