#include <bits/stdc++.h>
#include <cmath>

using namespace std;
typedef long long ll;
//typedef unsigned long long ll;

#define rep(i, n) for (int i = 0; i < (n); ++i)
//#define rep(i, n) for (ll i = 0; i < (n); ++i)
//#define sz(x) ll(x.size())
//typedef pair<ll, int> P;
typedef pair<ll, ll> P;
//const double INF = 1e10;
const ll INF = LONG_LONG_MAX / 100;
//const ll INF = (1ll << 31) - 1;
//const ll INF = 1e15;
const ll MINF = LONG_LONG_MIN;
//const int INF = INT_MAX / 10;
#define cmin(x, y) x = min(x, y)
#define cmax(x, y) x = max(x, y)
//typedef pair<int, int> P;
//typedef pair<double, double> P;

bool contain(set<P> &s, P a) { return s.find(a) != s.end(); }

//ifstream myfile("C:\\Users\\riku\\Downloads\\0_00.txt");
//ofstream outfile("log.txt");
//outfile << setw(6) << setfill('0') << prefecture << setw(6) << setfill('0') << rank << endl;
// std::cout << std::bitset<8>(9);
const int mod = 1000000007;
typedef priority_queue<long long, vector<long long>, greater<long long>> PQ_ASK;


void print_line(vector<string> &line) {
    if (line.size() == 0ll) {
        cout << endl;
        return;
    }
    for (ll i = 0; i < line.size(); i++) {
        cout << line[i];
        if (i == line.size() - 1) cout << endl;
        else cout << ' ';
    }
}

#define EPS 1e-7
const double PI = 3.14159265358979323846;

bool equal(double f1, double f2) {
    return abs(f1 - f2) < EPS;
}

struct Point;

struct Vec {

    double x, y;

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

    double inner_product(Vec v2) {
        return x * v2.x + y * v2.y;
    }

    Vec product(double rate) {
        Vec v2;
        v2.x = x * rate;
        v2.y = y * rate;
        return v2;
    }

    double outer_product(Vec v2) {
        return x * v2.y - v2.x * y;
    }

};


struct Point {
    double x, y;

    Point plus(Vec vec) {
        Point p;
        p.x = x + vec.x;
        p.y = y + vec.y;
        return p;
    }

    Point minus(Point sub) {
        Point p;
        p.x = x - sub.x;
        p.y = y - sub.y;
        return p;
    }

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

    friend istream &operator>>(istream &is, Point &f) {
        cin >> f.x >> f.y;
        return is;
    }

    friend ostream &operator<<(std::ostream &os, Point const &f) {
        printf("%.20f %.20f\n", f.x, f.y);
        return os;
    }

    Vec to_vec() {
        Vec v;
        v.x = x;
        v.y = y;
        return v;
    }
};

struct Line {
    Point start, end;

    Vec to_vec() {
        Vec vec;
        vec.x = end.x - start.x;
        vec.y = end.y - start.y;
        return vec;
    }

    friend istream &operator>>(istream &is, Line &f) {
        cin >> f.start >> f.end;
        return is;
    }

    Line normalize() {
        Line l;
        l.start = {0, 0};
        l.end = end.minus(start);
        return l;
    }

    double atan2() {
        Line l = normalize();
        return std::atan2(l.end.y, l.end.x);
    }

    double rad(Line line) {
        double f1 = atan2(),
                f2 = line.atan2();

        double diff = f1 - f2;
        if (equal(diff, -PI)) {
            return PI;
        }
        if (equal(diff, PI)) {
            return PI;
        }
        if (diff < -PI) {
            diff += (PI * 2);
        }
        if (diff > PI) {
            diff -= (PI * 2);
        }

        return diff;
    }

    void clock_wise(Line line) {

        double diff = rad(line);

        if (equal(line.to_vec().size(), 0)) {
            cout << "ON_SEGMENT" << endl;
            return;
        }

        if (equal(diff, 0)) {
            double s1 = to_vec().size();
            double s2 = line.to_vec().size();

            if (s1 > s2 || equal(s1, s2)) {
                cout << "ON_SEGMENT" << endl;
            } else {
                cout << "ONLINE_FRONT" << endl;
            }
        } else if (equal(abs(diff), PI)) {
            cout << "ONLINE_BACK" << endl;
        } else if (diff < 0) {
            cout << "COUNTER_CLOCKWISE" << endl;
        } else {
            cout << "CLOCKWISE" << endl;
        }
    }

    static const int COUNTER_CLOCKWISE = 1;
    static const int CLOCKWISE = -1;
    static const int ONLINE_BACK = 2;   // p2->p0->p1
    static const int ONLINE_FRONT = -2; // p0->p1->p2
    static const int ON_SEGMENT = 0;    // p0->p2->p1

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

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

    double norm(Point a) {
        return a.x * a.x + a.y * a.y;
    }

    int ccw(Point p0, Point p1, Point p2) {
        Point a = p1.minus(p0);
        Point b = p2.minus(p0);
        if (cross(a, b) > EPS) return COUNTER_CLOCKWISE;
        if (cross(a, b) < -EPS) return CLOCKWISE;
        if (dot(a, b) < -EPS) return ONLINE_BACK;
        if (norm(a) < norm(b)) return ONLINE_FRONT;
        return ON_SEGMENT;
    }

    bool intersect(Point p1, Point p2, Point p3, Point p4) {
        return (ccw(p1, p2, p3) * ccw(p1, p2, p4) <= 0 &&
                ccw(p3, p4, p1) * ccw(p3, p4, p2) <= 0);
    }

    bool intersect(Line s1) {
        return intersect(start, end, s1.start, s1.end);
    }

    Point cross_point(Line s1) {
        Point base = s1.end.minus(s1.start);
        double d1 = abs(cross(base, start.minus(s1.start)));
        double d2 = abs(cross(base, end.minus(s1.start)));
        double t = d1 / (d1 + d2);
        Point e = end.minus(start);
        Line l;
        l.start = {0, 0};
        l.end = e;
        return start.plus(l.to_vec().product(t));
    }

    double distance_lp(Line l, Point p) {
        return abs(cross(l.end.minus(l.start), p.minus(l.start)) / l.end.minus(l.start).distance());
    }

    double distance_sp(Line s, Point p) {
        if (dot(s.end.minus(s.start), p.minus(s.start)) < 0.0) return p.minus(s.start).distance();
        if (dot(s.start.minus(s.end), p.minus(s.end)) < 0.0) return p.minus(s.end).distance();
        return distance_lp(s, p);
    }

    double distance(Line s2) {
        if (intersect(s2)) return 0.0;
        return min({distance_sp(*this, s2.start), distance_sp(*this, s2.end),
                    distance_sp(s2, start), distance_sp(s2, end)});
    }
};


int main() {
    int n;
    cin >> n;


    vector<Point> points;
    rep(i, n) {
        Point p;
        cin >> p;
        points.push_back(p);
    }
    double ans = 0;
    rep(i, n) {
        int j = (i + 1) % n;

        Vec v1 = points[i].to_vec(), v2 = points[j].to_vec();

        double now = v1.outer_product(v2);
        ans += now;

    }
    ans /= 2;
    ans = abs(ans);
    printf("%.1f\n", ans);
}



Measured memory_bytes: 3268