#include <iostream>
#include <vector>
#include <iomanip>
#include <cmath>

using namespace std;

const double EPS = 1e-8;

bool dcmp(double d) {
    if (fabs(d) < EPS) {
        return 0;
    }
    else {
        return d < 0? -1 : 1;
    }
}

struct Vector {
    double x, y;
    Vector(double x = 0, double  y = 0):x(x), y(y) {
    }
};

typedef Vector Point;

Vector operator-(const Vector& a, const Vector& b) {
    return Vector(a.x - b.x, a.y - b.y);
}

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

double ccw(const Point& a, const Point& b, const Point& c) {
    return cross(b - a, c - a);
}

int main() {
    int n; 
    cin >> n;
    vector<Point> p(n);
    for (int i = 0; i < n; i++) {
        cin >> p[i].x >> p[i].y;
    }
    
    double area = 0;
    for (int i = 1; i < n - 1; i++) {
        area += ccw(p[0], p[i], p[i+1]) / 2;
    }
    
    cout << fixed << setprecision(1) << area << endl;
    
    return 0;
}