#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <map>
#include <cmath>
#include <iomanip>
#include <complex>
using namespace std;
#define REP(i,n) for (int i=0;i<(n);++i)
#define rep(i,a,b) for(int i=a;i<(b);++i)
template<class T> inline bool chmin(T &a, T b){ if(a > b) { a = b; return true;} return false;}
template<class T> inline bool chmax(T &a, T b){ if(a < b) { a = b; return true;} return false;}
using ll = long long;
constexpr long long INF = 1LL << 62;
constexpr int MOD = 1e9 + 7;


constexpr double EPS = 1e-10;
using Point = complex<double>;
using Line  = pair<Point, Point>;
using Polygon = vector<Point>;
bool compare(const Point &a, const Point &b) { // x ascending order
    if(a.real() != b.real()) return a.real() < b.real();
    return a.imag() < b.imag();
};

double dot(Point p, Point q) { return (conj(p) * q).real(); }
double cross(Point p, Point q) { return (conj(p) * q).imag(); }
double slope(Line l) { return tan(arg(l.second - l.first)); }

Point project(Line l, Point p) { // project p onto line (s,t)
    return l.first + (l.second - l.first) * dot(p - l.first, l.second - l.first) / norm(l.second - l.first);
}

Point reflect(Line l, Point p) {
    return l.first + conj((p - l.first) / (l.second - l.first)) * (l.second - l.first);
}

int ccw(Point a, Point b, Point c) {
    b -= a; c -= a;
    if(cross(b, c) >  EPS) return +1; // counter-clockwise
    if(cross(b, c) < -EPS) return -1; // clockwise
    if(  dot(b, c) < -EPS) return +2; // c--a--b
    if(abs(b)+EPS < abs(c))return -2; // a--b--c
    return 0; // a--c--b
}

bool intersect(Line a, Line b) {
    return ccw(a.first, a.second, b.first) * ccw(a.first, a.second, b.second) <= 0 && 
        ccw(b.first, b.second, a.first) * ccw(b.first, b.second, a.second) <= 0;
}

Point cross_point(Line a, Line b) {
    Point base = b.second - b.first;
    double area1 = abs(cross(base, a.first - b.first));
    double area2 = abs(cross(base, a.second - b.first));
    double t = area1 / (area1 + area2);
    return a.first + (a.second - a.first) * t;
}

// dist line and p
double distance(Point p, Line l) { return abs(cross(l.second - l.first, p - l.first) / abs(l.second - l.first)); }
double distance(Line s, Point p) { 
    Point a = s.first, b = s.second;

    if(dot(b - a, p - a) < EPS) return abs(p - a);
    if(dot(a - b, p - b) < EPS) return abs(p - b);
    return distance(p, s);
}
double distance(Line s1, Line s2) {
    if(intersect(s1, s2)) return 0.0;
    return min(min(distance(s1, s2.first), distance(s1, s2.second)), min(distance(s2, s1.first), distance(s2, s1.second)));
}

double area(Polygon &ps) {
    double res = 0.0;
    for(int i = 0; i < (int)ps.size(); ++i) 
        res += cross(ps[i], ps[(i+1)%(int)ps.size()]);
    return abs(res) / 2.0;
}
    

Polygon convex_hull(Polygon &ps) {
    sort(ps.begin(), ps.end(), compare);
    int n = (int)ps.size(), k = 0;
    Polygon res(n * 2);

    // use "< -EPS" if include corner or boundary, otherwise, use "< EPS"
    for(int i = 0; i < n; res[k++] = ps[i++]) 
        while(k >= 2 && cross(res[k-1] - res[k-2], ps[i] - res[k-1]) < EPS) --k; 

    for(int i = n - 2, t = k+1; i >= 0; res[k++] = ps[i--])
        while(k >= t && cross(res[k-1] - res[k-2], ps[i] - res[k-1]) < EPS) --k;

    res.resize(k - 1);
    return res;
}

double caliper(Polygon &ps) {
    ps = convex_hull(ps);
    int n = (int)ps.size();
    if(n == 2) {return abs(ps[0] - ps[1]);}

    int i = 0, j = 0;
    // j --> (x asc order) --> i
    for(int k=0; k < n; ++k) {
        if(compare(ps[i],ps[k])) i = k;
        if(compare(ps[k],ps[j])) j = k;
    }

    int si = i, sj = j;
    double res = 0.0;
    // rotate 180 degrees
    while(i != sj || j != si) {
        res = max(res, abs(ps[i] - ps[j]));
        if(cross(ps[(i + 1) % n] - ps[i], ps[(j + 1) % n] - ps[j]) < 0) {
            i = (i + 1) % n;
        } else {
            j = (j + 1) % n;
        }
    }

    return res;
}


int main() {
    cin.tie(0); ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(1);
    int q; cin >> q;
    Polygon ps;
    for(int i=0; i < q; ++i) {
        double x,y; cin >> x >> y;
        ps.emplace_back(x,y);
    }
    cout << area(ps) << endl;
    return 0;
}



Measured memory_bytes: 3324