#include <vector>
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <climits>

using namespace std;

#define EPS (1e-10)
#define equals(a, b) (fabs((a) - (b)) < EPS)

//???
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 norm(){return x*x + y*y;};
    double absolute(){return sqrt(norm());};
    
    bool operator < (const Point &p) const{
        return x != p.x ? x < p.x : y < p.y;
    }
    
    bool operator == (const Point &p) const{
        return equals(x, p.x) && equals(y, p.y);
    }
    
};
typedef Point Vector;                               //????????????
typedef struct Segment{Point p1, p2;}Segment;       //??????
typedef Segment Line;                               //??´???
typedef struct Circle{Point c; double r;}Circle;    //???
typedef vector<Point> Polygon;                      //????§???¢
//????????????a????????????
double norm(Vector a){
    return a.x * a.x + a.y * a.y;
}
//????????????a?????§??????
double absolute(Vector a){
    return sqrt(norm(a));
}

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

//????????????a??¨b?????´?????????
//a??¨b?????´????????´??????????????????0?????????
bool isOrthogonal(Vector a, Vector b){
    return equals(dot(a,b), 0.0);
}

//????????????a??¨b???????????????
//a??¨b?????´????????´??????????????????0?????????
bool isParallel(Vector a , Vector b){
    return equals(cross(a,b), 0.0);
}

//??????s(p1p2)???????????????p????°???±
Point project(Segment s, Point p){
    Vector base = s.p2 - s.p1;
    double r = dot(p - s.p1, base) / norm(base);
    return s.p1 + base * r;
}

//??????s(p1p2)???????§°?????¨?????????p???????±????
Point reflect(Segment s, Point p){
    return p + (project(s, p) - p) * 2.0;
}

//???a,???b???????????¢
double getDistance(Point a, Point b){
    return absolute(a - b);
}

//??´???s??¨???p????????¢
double getDistanceLP(Line l, Point p){
    return abs(cross(l.p2 - l.p1, p - l.p1) / absolute(l.p2 - l.p1));
}

//??????s(p1p2)??¨???p????????¢d????±???????
//??\???????????????????????§??´?????????
//??????????????????p2-p1??¨???????????????p-p1??????????§???????90???????????§????????´???????????????-90?????????????°?????????´?????????d??????p??¨???p1??¨????????¢
//??????????????????p1-p2??¨???????????????p-p2??????????§???????90???????????§????????´???????????????-90?????????????°?????????´?????????d??????p??¨???p2??¨????????¢
//??????????????\???????????????????????´??????d??????p??¨??´???p1p2????????¢
double getDistanceSP(Segment s, Point p){
    if (dot(s.p2 - s.p1, p - s.p1) < 0.0) return absolute(p - s.p1);
    if (dot(s.p1 - s.p2, p - s.p2) < 0.0) return absolute(p - s.p2);
    return getDistanceLP(s, p);
}


//??????p0, p1, p2???????????????????????????p0p1???????????????p2??????????????????
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(norm(a) < norm(b)) return ONLINE_FRONT;
    return ON_SEGMENT;
}

//??????S1(p1p2)??¨??????s2(p2p3)????????????????????????
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(Segment s1, Segment s2){
    return intersect(s1.p1, s1.p2, s2.p1, s2.p1);
}

//??????s1??¨??????s2????????¢
//??\????????????????????¢????????§????????????????????????
//????????????s1??¨??????s2????§????s2.p1????????¢
//????????????s1??¨??????s2?????????s2.p2????????¢
//????????????s2??¨??????s1????§????s1.p1????????¢
//????????????s2??¨??????s1?????????s1.p2????????¢
double getDistance(Segment s1, Segment s2){
    if ( intersect(s1, s2) ) return 0.0;
    return min(
               min(getDistanceSP(s1, s2.p1), getDistanceSP(s1, s2.p2)),
               min(getDistanceSP(s2, s1.p1), getDistanceSP(s2, s1.p2))
               );
}

//??????s1??¨??????s2?????????????±???????
Point getCrossPoint(Segment s1, Segment s2){
    Vector base = s2.p2 - s2.p1;
    double d1 = abs(cross(base, s1.p1 - s2.p1));
    double d2 = abs(cross(base, s1.p2 - s2.p1));
    double t = d1 / (d1 + d2);
    return s1.p1 + (s1.p2 - s1.p1) * t;
}

//?????¨??????l?????????????±???????
pair<Point, Point> getCrossPoints(Circle c, Line l){
    Vector pr = project(l, c.c);
    Vector e = (l.p2 - l.p1) / absolute(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?????????????±???????
Vector polar(double a, double r){
    //??????a,?§????r???point????±???????
    return Point(cos(r) * a, sin(r) * a);
}
pair<Point, Point> getCrossPoints(Circle c1, Circle c2){
    Vector v = c2.c - c1.c;
    double d = absolute(v);
    double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d) );
    double t = atan2(v.y, v.x);
    return make_pair(c1.c + polar(c1.r, t+a), c1.c + polar(c1.r, t-a));
}


//http://imagingsolution.net/math/calc_n_point_area/
int main(){
    
    int n;
    double ans = 0.0;
    double area = 0.0, s = 0.0, t = 0.0;
    
    cin >> n;
    
    Vector *q = new Vector[n];
    
    for (int i = 0; i < n; i++) {
        cin >> q[i].x;
        cin >> q[i].y;
    }
    
    for (int i = 0; i < n; i++) {
        if ( ( area = cross(q[i], q[(i+1)%n]) ) > EPS) {
            s += fabs(area) / 2.0;
        }
        else{
            t += fabs(area) / 2.0;
        }
    }
    ans = fabs(s-t);
    
    printf("%.1lf\n", ans);

    return 0;
}

Measured memory_bytes: 1248