#include<bits/stdc++.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) for(int i = 0; i < (b); i++)
#define all(a) (a).begin(), (a).end()
#define show(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
const int INF = 2000000000;
using namespace std;

typedef complex<double> Point;
typedef Point Vector;
//???????????¨????§???????
struct Segment{ Point p1, p2; };
//??´????????¨????§???????
typedef Segment Line;
//????§???¢?????¨????§???????
typedef vector<Point> Polygon;

namespace std{
    bool operator < (const Point &a, const Point &b){
        return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
    }
    bool operator == (const Point &a, const Point &b){
        return a.real() == b.real() && a.imag() == b.imag();
    }
}

class Circle{
    public:
        Point c;
        double r;
        Circle(Point c = Point(), double r = 0.0): c(c), r(r) {}
};

// ?¨±???????????????
#define EPS (1e-10)

// ????????????a?????¶??????????±???????
//double length = abs(a);

// 2???a,b???????????¢????±???????
//double distance = abs(a-b);

/*
// ????????????a?????????????????????????±???????
Point b = a / abs(a);

// ????????????a?????????????????????n1,n2????±???????
Point n1 = a * Point(0, 1);
Point n2 = a * Point(0, -1);
*/

int ccw(Point, Point, Point);

// 2??????????????????????????????????????????
bool EQ(double a, double b){
    return (abs(a - b) < EPS);
}

// 2??????????????????????????????????????????
bool EQV(Vector a, Vector b){
    return ( EQ(a.real(), b.real()) && EQ(a.imag(), b.imag()) );
}

// ?????? (dot product) : a???b = |a||b|cos??
double dot(Point a, Point b) {
    return (a.real() * b.real() + a.imag() * b.imag());
}

// ?????? (cross product) : a??b = |a||b|sin??
double cross(Point a, Point b) {
    return (a.real() * b.imag() - a.imag() * b.real());
}

// 2??´????????´????????? : a??\b <=> dot(a, b) = 0
bool isOrthogonal(Point a1, Point a2, Point b1, Point b2) {
    return EQ( dot(a1-a2, b1-b2), 0.0 );
}
bool isOrthogonal(Line s1, Line s2) {
    return isOrthogonal(s1.p1, s1.p2, s2.p1, s2.p2);
}

// 2??´?????????????????? : a//b <=> cross(a, b) = 0
bool isParallel(Point a1, Point a2, Point b1, Point b2) {
    return EQ( cross(a1-a2, b1-b2), 0.0 );
}
bool isParallel(Line s1, Line s2) {
    return isParallel(s1.p1, s1.p2, s2.p1, s2.p2);
}

// ???c?????´???a,b????????????????????????
bool isPointOnLine(Point a, Point b, Point c) {
    return EQ( cross(b-a, c-a), 0.0 );
}
bool isPointOnLine(Line s, Point c) {
    return isPointOnLine(s.p1, s.p2, c);
}

// ???a,b???????????´?????¨???c??¨????????¢
double distanceLPoint(Point a, Point b, Point c) {
    return abs(cross(b-a, c-a)) / abs(b-a);
}
double distanceLPoint(Line s, Point c) {
    return distanceLPoint(s.p1, s.p2, c);
}

// ???a,b???????????¨??????????????¨???c??¨????????¢
double distanceLsPoint(Point a, Point b, Point c) {
    if ( dot(b-a, c-a) < EPS ) return abs(c-a);
    if ( dot(a-b, c-b) < EPS ) return abs(c-b);
    return abs(cross(b-a, c-a)) / abs(b-a);
}
double distanceLsPoint(Segment s, Point c) {
    return distanceLsPoint(s.p1, s.p2, c);
}

// a1,a2???????????¨??????????????¨b1,b2???????????¨???????????????????????????
bool isIntersectedLs(Point a1, Point a2, Point b1, Point b2) {
    return ( ccw(a1, a2, b1) * ccw(a1, a2, b2) <= 0 &&
            ccw(b1, b2, a1) * ccw(b1, b2, a2) <= 0 );
}
bool isIntersectedLs(Segment s1, Segment s2) {
    return isIntersectedLs(s1.p1, s1.p2, s2.p1, s2.p2);
}

// a1,a2???????????¨??????????????¨b1,b2???????????¨??????????????????????¨????
Point intersectionLs(Point a1, Point a2, Point b1, Point b2) {
    Vector base = b2 - b1;
    double d1 = abs(cross(base, a1 - b1));
    double d2 = abs(cross(base, a2 - b1));
    double t = d1 / (d1 + d2);

    return Point(a1 + (a2 - a1) * t);
}
Point intersectionLs(Segment s1, Segment s2) {
    return intersectionLs(s1.p1, s1.p2, s2.p1, s2.p2);
}

// a1,a2???????????´?????¨b1,b2???????????´??????????????????
bool isIntersectedL(Point a1, Point a2, Point b1, Point b2) {
    return !EQ( cross(a1-a2, b1-b2), 0.0 );
}
bool isIntersectedL(Line l1, Line l2) {
    return isIntersectedL(l1.p1, l1.p2, l2.p1, l2.p2);
}

// a1,a2???????????´?????¨b1,b2???????????´?????????????¨????
Point intersectionL(Point a1, Point a2, Point b1, Point b2) {
    Point a = a2 - a1; Point b = b2 - b1;
    return a1 + a * cross(b, b1-a1) / cross(b, a);
}
Point intersectionL(Line l1, Line l2) {
    return intersectionL(l1.p1, l1.p2, l2.p1, l2.p2);
}

// ??????s1??¨??????s2????????¢
double distanceLL(Segment s1, Segment s2){
    if(isIntersectedLs(s1.p1, s1.p2, s2.p1, s2.p2) ) return 0.0;
    return min(
            min(distanceLsPoint(s1.p1, s1.p2, s2.p1),
                distanceLsPoint(s1.p1, s1.p2, s2.p2)),
            min(distanceLsPoint(s2.p1, s2.p2, s1.p1),
                distanceLsPoint(s2.p1, s2.p2, s1.p2)) );
}

// ??????s???????????????p????°???±
Point project(Segment s, Point p){
    Vector base = s.p2 - s.p1;
    double r = dot(p - s.p1, base) / norm(base);
    return Point(s.p1 + base * r);
}

//??????s???????±??????¨?????????p??????????§°??????
Point reflect(Segment s, Point p){
    return Point(p + (project(s, p) - p) * 2.0);
}

//???p???angle?????????????¨???????????????¢
Point rotation(Point p, double angle){
    double x, y;
    x = p.real() * cos(angle) - p.imag() * sin(angle);
    y = p.real() * sin(angle) + p.imag() * cos(angle);
    return Point(x, y);
}

//???c??¨??????l?????????
pair<Point, Point> getCrossPoints(Circle c, Line l){
    Vector pr = project(l, c.c);
    Vector e = (l.p2 - l.p1) / abs(l.p2 - l.p1);
    double base = sqrt(c.r * c.r - norm(pr - c.c));
    return make_pair(pr + e * base, pr - e * base);
}

//???c1??¨???c2?????????
double arg(Vector p) { return atan2(p.imag(), p.real()); }
Vector polar(double a, double r) { return Point(cos(r) * a, sin(r) *a); }

pair<Point, Point> getCrossPoints(Circle c1, Circle c2){
    double d = abs(c1.c - c2.c);
    double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d));
    double t = arg(c2.c - c1.c);
    return make_pair(Point(c1.c + polar(c1.r, t + a)), Point(c1.c + polar(c1.r, t - a)));
}

//????????????
static const int IN = 2;
static const int ON = 1;
static const int OUT = 0;

int contains(Polygon g, Point p){
    int n = g.size();
    bool x = false;
    rep(i,n){
        Point a = g[i] - p, b = g[(i + 1) % n] - p;
        if( abs(cross(a, b)) < EPS && dot(a,  b) < EPS ) return ON;
        if( a.imag() > b.imag() ) swap(a, b);
        if( a.imag() < EPS && EPS < b.imag() && cross(a, b) > EPS ) x = not x;
    }
    return ( x ? IN : OUT );
}

//???????????????????????????
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;

int ccw(Point p0, Point p1, Point p2){
    Vector a = p1 - p0;
    Vector b = p2 - 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( abs(a) < abs(b) ) return ONLINE_FRONT;

    return ON_SEGMENT;
}

//??????
Polygon andrewScan( Polygon s ){
    Polygon u, l;
    if( s.size() < 3 ) return s;
    sort(s.begin(), s.end());

    u.emplace_back(s[0]);
    u.emplace_back(s[1]);

    l.emplace_back(s[s.size() - 1]);
    l.emplace_back(s[s.size() - 2]);

    range(i,2,s.size()){
        //== COUNTER_CLOCKWISE?????¨????§????180??\???????????´?????????????????§????????????????????¨????????°?????????
        //!= CLOCKWISE?????¨????§????180??????????????´????????????????????°????????????
        for(int n = u.size(); n >= 2 && ccw(u[n-2], u[n-1], s[i]) != CLOCKWISE; n--){
            u.pop_back();
        }
        u.emplace_back(s[i]);
    }

    for(int i = s.size() - 3; i >= 0; i--){
        for(int n = l.size(); n >= 2 && ccw(l[n-2], l[n-1], s[i]) != CLOCKWISE; n--){
            l.pop_back();
        }
        l.emplace_back(s[i]);
    }

    reverse(l.begin(), l.end());
    for(int i = u.size() - 2; i >= 1; i--) l.emplace_back(u[i]);

    return l;
}


double AreaOfTriangle(Point a, Point b, Point c){
    double w, x, y, z;
    w = b.real()-a.real();
    x = b.imag()-a.imag();
    y = c.real()-a.real();
    z = c.imag()-a.imag();
    return abs((w * z - x * y) / 2);
}

double areaOfPolygon(Polygon g){
    int n = g.size();
    double ret = 0.0;
    rep(i,n) ret += cross(g[i], g[ (i + 1) % n ]);
    return abs(ret) / 2.0;
}

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

    Polygon g;
    rep(i,n){
        double a, b;
        cin >> a >> b;
        g.emplace_back(Point{a,b});
    }
    cout << fixed << setprecision(1) << areaOfPolygon(g) << endl;
}

Measured memory_bytes: 3300