#include<bits/stdc++.h>
using namespace std;
#define r(i,n) for(int i=0;i<n;i++)
#define R(i,a,n) for(int i=a;i<n;i++)
#define EPS (1e-10)
#define equals(a,b) (fabs((a)-(b))<EPS)
//CCW??¨///////////////////////////////////
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;
/////////////////////////////////////////
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);}

  bool operator < (const Point &p) const{
    return x!=p.x?x<p.x:y<p.y;
  }
  bool operator == (const Point &p) const{
    return fabs(x-p.x)<EPS&&fabs(y-p.y)<EPS;
  }
};
struct Circle{
  Point c;
  double r;
};
typedef Point vect;
struct seg{Point p1,p2;};
//????????´????????????
double norm(Point p){return p.x*p.x+p.y*p.y;}
//??¶???????????????absolute  ?????????????????¶?????????
double abs(Point p){return sqrt(norm(p));}
//?????????????????????????????????
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;
}
//??´?????????????????¨?????\?????????????????????
bool C90(seg s1,seg s2){
  return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}
//????????????????????¨?????\??????????????????????????????????????????
bool C0(seg s1,seg s2){
  return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}
//?°???± ??????????????´???????????????
Point project(seg s,Point p){
  Point base = s.p2-s.p1;
  double r=dot(p-s.p1,base) / norm(base);
  return s.p1+base*r;
}
//????°??????????????????????
Point reflection(seg s,Point p){
  return  p+(project(s,p)-p)*2.0;
}
//2???????????¢   ???????????????
double getDistancePP(Point a,Point b){
  return abs(a-b);
}
//??´??????????????¢(????°?)???????????????
double getDistanceLP(seg l,Point p){
  return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}
//?????????????????¢??????????????????
double getDistanceSP(seg s,Point p){
  if(dot(s.p2-s.p1,p-s.p1)<0.0)return abs(p-s.p1);
  if(dot(s.p1-s.p2,p-s.p2)<0.0)return abs(p-s.p2);
  return getDistanceLP(s,p);
}
//????¨?????????????????¨??????????????????????????????????
int CCW(Point p0,Point p1,Point p2){
  Point a=p1-p0;
  Point 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;
}
//????????????????????????????????????????????§?\????
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(seg s1,seg s2){
  return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}
//2??????????§???¢?????\??????????????????...????????????????????????
bool squareintersect(seg s1,seg s2){
  if(s1.p2.x<s2.p1.x||s2.p2.x<s1.p1.x)return 0;
  if(s1.p2.y<s2.p1.y||s2.p2.y<s1.p1.y)return 0;
  return 1;
}
//??????????????¢?????????????????????????????§??¨???????????????
double getDistance(seg s1,seg 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)));
}
//??????????????????????????????(?´????????`)
Point getCrossPoint(seg s1,seg s2){
  Point 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;
}
//???????£????????????¢??????????????¢??????????????????????????\????????????????????§???????????£???????????????????????????????????§?????????????????????
int intersectCC(Circle a,Circle b){
  double dist=abs(a.c-b.c);
  if(dist>a.r+b.r+EPS)return 4;
  if(dist>a.r+b.r-EPS)return 3;
  if(dist>abs(a.r-b.r)+EPS)return 2;
  if(dist>abs(a.r-b.r)-EPS)return 1;
  return 0;
}
//?????¨??´???????????????2?????? (LINE)
seg getCrossPoint(Circle c,seg l){
  //assert(intersect(cc,l));
  Point pr=project(l,c.c);
  Point e=(l.p2-l.p1)/abs(l.p2-l.p1);
  double base =sqrt(c.r*c.r-norm(pr-c.c));
  seg pp;pp.p1=(pr+e*base);pp.p2=(pr-e*base);
  return pp;
}
//?????¨??´??????Line??????????????°
int getCircleLine(Circle c,seg l){
  seg a=getCrossPoint(c,l);
  if(isnan(a.p1.x)&&isnan(a.p2.y))return 0;
  else if(a.p1.x==a.p2.x&&a.p1.y==a.p2.y)return 1;
  else return 2;
}
//??????
Point gaishin(Point a, Point b, Point c){
  double a1,a2,b1,b2,c1,c2;
  a1=2*(b.x-a.x);b1=2*(b.y-a.y);c1=a.x*a.x-b.x*b.x+a.y*a.y-b.y*b.y;
  a2=2*(c.x-a.x);b2=2*(c.y-a.y);c2=a.x*a.x-c.x*c.x+a.y*a.y-c.y*c.y;
  Point p;p.x=(b1*c2-b2*c1)/(a1*b2-a2*b1);p.y=(c1*a2-c2*a1)/(a1*b2-a2*b1);
  return p;
}
//2??????????????????
double arg(Point p){return atan2(p.y,p.x);}
Point polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}
seg getCrossPoints(Circle c1,Circle c2){
  //assert(intersect(c1,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);
  seg s;s.p2=c1.c+polar(c1.r,t+a);s.p1=c1.c+polar(c1.r,t-a);
  return s;
}
//???????????? ????????????->2 ??????->1  ????????\???->0
typedef vector<Point> Polygon;
int contains(Polygon g, Point p) {
  int n=g.size();
  bool x=false;
  for(int i=0;i<n;i++) {
    Point a=g[i]-p,b=g[(i+1)%n]-p;
    if(abs(cross(a, b)) < EPS && dot(a, b) < EPS)return 1;
    if(a.y>b.y)swap(a,b);
    if(a.y<EPS&&EPS<b.y&&cross(a,b)>EPS)x=!x;
  }
  return x ? 2 : 0;
}
//?????¢?????¢???
double Area(Polygon p) {
  double a=0;
  for(int i=0;i<p.size();i++)a+=cross(p[i],p[(i+1)%p.size()]);
  return a/2;
}
int main(){
  int n;
  Polygon p;
  Point s;
  cin>>n;
  while(n--){
    cin>>s.x>>s.y;
    p.push_back(s);
  }
  printf("%.1f\n",Area(p));
}

Measured memory_bytes: 3280