#include<iostream>
#include<map>
#include<vector>
#include<algorithm>
#include<cmath>
#include<climits>
#include<ctime>
#include<cstring>
#include<stack>
#include<queue>
#include<sstream>
#include<string>
#include<set>
#include<array>
#include<cassert>

#define ALL(v) (v).begin(),(v).end()
#define REP(i,p,n) for(int i=p;i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define DUMP(list) cout << "{ "; for(auto nth : list){ cout << nth << " "; } cout << "}" << endl
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i);

using namespace std;

typedef double Real;

const Real EPS = 1e-8;

int sign(Real d){
	return d > EPS ? 1 : d < -EPS ? -1 : 0;
}

struct Point{
	Real x,y;	
	explicit Point(Real x_ = 0,Real y_ = 0):x(x_),y(y_){}
	Point operator+(const Point &p) const {return Point(x+p.x,y+p.y);}
	Point operator-(const Point &p) const {return Point(x-p.x,y-p.y);}
	Point operator*(Real s) const {return Point(x*s,y*s);}
	Point operator/(Real s) const {return Point(x/s,y/s);}
	bool operator<(const Point &p) const{return sign(x-p.x) == -1 || (sign(x-p.x) == 0 && sign(y-p.y) == -1);}
	bool operator==(const Point &p) const{return sign(x-p.x) == 0 && sign(y-p.y) == 0;}

};

istream &operator>>(istream &is,Point &p){return is >> p.x >> p.y;}//??\???????°???????
ostream &operator<<(ostream &os ,const Point &p){return os << '(' << p.x << ", " << p.y << ')';}//??????????°???????

struct Segment : public array<Point,2>{
	Segment(const Point &a,const Point &b){	at(0) = a; at(1) = b;}
};

struct Line : public array<Point,2>{
	Line(const Point &a,const Point &b){at(0) = a; at(1) = b;}
};

struct Circle{
	Point c;
	Real r;

	Circle(const Point &c_, Real r_):c(c_),r(r_){}
};

typedef vector<Point> Polygon;

Point rotate90(const Point &p){
	return Point(-p.y,p.x);
}

Point rotate(const Point &p,Real theta){
	const Real s = sin(theta),c = cos(theta);
	return Point(c*p.x-s*p.y,s*p.x+c*p.y);
}

Real angle(const Point &p){
	return atan2(p.y,p.x);
}

Real dot(const Point &a,const Point &b){ //??????????????????
	return a.x*b.x+a.y*b.y;
}

Real cross(const Point &a,const Point &b){ //??????????????????
	return a.x*b.y-a.y*b.x;
}


Real norm(const Point &p){
	return p.x*p.x+p.y*p.y;
}

Real abs(const Point &p){
	return sqrt(norm(p));
}

enum { CCW=1,CW=-1,BACK=2,FRONT=-2,ON=0 };
int ccw(const Point &a,const Point &b,const Point &c){
	const Point p = b-a;
	const Point q = c-a;
	const int sgn = sign(cross(p,q));
	
	if(sgn == 1) return CCW;
	if(sgn == -1) return CW;
	if(sign(dot(p,q)) == -1) return BACK;
	if(sign(norm(p) - norm(q)) == -1) return FRONT;
	return ON;
}

Point project(const Line &l,const Point &p){ //?°???±
	Real t = dot(p-l[0], l[1]-l[0]) / norm(l[0] - l[1]);
	return l[0] + (l[1] - l[0]) * t;
}

Point refrect(const Line &l,const Point &p){ //????°?
	const Point c = project(l,p);
	return c+(c-p);
}

bool intersect(const Segment &a,const Segment &b){
	return ccw(a[0],a[1],b[0])*ccw(a[0],a[1],b[1]) <= 0
		&& ccw(b[0],b[1],a[0])*ccw(b[0],b[1],a[1]) <= 0;
}

bool intersect(const Segment &s,const Point &p){
	return ccw(s[0],s[1],p) == ON;
}

bool intersect(const Line &l,const Segment &s){
	return sign(cross(l[1]-l[0],s[0]-l[0])) * cross(l[1]-l[0],s[1]-l[0]) <= 0;
}

bool intersect(const Line &l,const Point &p){
	return abs(ccw(l[0],l[1],p)) != 1;
}

bool intersect(const Line &a,const Line &b){ //????????????
	return sign(cross(a[1]-a[0],b[1]-b[0])) != 0
		||	sign(cross(a[1]-a[0],b[1]-a[0]) == 0);
}

Real dist(const Point &a,const Point &b){
	return abs(a-b);	
}

Real dist(const Line &l,const Point &p){
	const Point a = l[1]-l[0];
	const Point b = p-l[0];
	return abs(cross(a,b))/abs(a);
}

Real dist(const Line &l,const Segment &s){
	if(intersect(l,s)) return 0;
	return min(dist(l,s[0]),dist(l,s[1]));
}

Real dist(const Line &a,const Line &b){
	if(intersect(a,b)) return 0;
	return dist(a,b[0]);
}

Real dist(const Segment &s,const Point &p){
	if(sign(dot(s[1]-s[0],p-s[0]))==-1) return dist(s[0],p);
	if(sign(dot(s[0]-s[1],p-s[1]))==-1) return dist(s[1],p);
	return dist(Line(s[0],s[1]),p);
}

Real dist(const Segment &a,const Segment &b){
	if(intersect(a,b)) return 0;
	return min({dist(a,b[0]),dist(a,b[1]),dist(b,a[0]),dist(b,a[1])});
}

bool intersect(const Circle &a,const Circle &b){
	return sign(dist(a.c,b.c)-(a.r+b.r)) <=0
			&& sign(dist(a.c,b.c)-abs(a.r-b.r)) >= 0;
}

bool intersect(const Circle &c,const Segment &s){
	return sign(dist(s,c.c)-c.r)<=0;
}

bool intersect(const Circle &c,const Line &l){
	return sign(dist(l,c.c)-c.r)<=0;
}

bool contain(const Circle &c,const Point &p){
	return sign(dist(c.c,p)-c.r) <=0;
}

bool contain(const Polygon &P,const Point &p){
	bool res = false;

	for(int i=0;i<P.size();++i){
		Point v1=P[i]-p;
		Point v2=P[(i+1)%P.size()]-p;
		if(v1.y>v2.y) swap(v1,v2);

		if(sign(cross(v1,v2))==0&&sign(dot(v1,v2))<=0){
			return true;//on edge 
		}
		if(sign(v1.y)<=0&&sign(v2.y)==1&&sign(cross(v1,v2))==1){
			res=!res;
		}
	}
	return res;
}

Point crosspoint(const Line &a,const Line &b){
	assert(intersect(a,b));
	const Real crs = cross(a[1]-a[0],b[1]-b[0]);
	if(sign(crs) == 0) return a[0];
	return b[0]+(b[1]-b[0])*(cross(a[1]-a[0],a[1]-b[0])/crs);
}


//??¬??¢?????????????????¨?????°?????????????????????Intersect?????????
Point crosspoint(const Segment &a,const Segment &b){
	assert(intersect(a,b));
	const Real crs = cross(a[1]-a[0],b[1]-b[0]);
	if(sign(crs)==0){
		if(intersect(a,b[0])) return b[0];
		if(intersect(a,b[1])) return b[1];
		if(intersect(b,a[0])) return a[0];
		return a[1];
	}
	return b[0]+(b[1]-b[0])*(cross(a[1]-a[0],a[1]-b[0])/crs);
}

vector<Point> crosspoint(const Circle &c,const Line &l){
	const Point p = project(l,c.c);
	const Real h = dist(p,c.c);

	vector<Point> res;
	if(sign(h-c.r)==1){
		//nothing
	}else if(sign(h-c.r)==0){
		res.emplace_back(p);	
	}else{
		const Real b = sqrt(c.r*c.r-h*h);
		const Point e = (l[1]-l[0])/abs(l[1]-l[0]);
		res.emplace_back(p+e*b);
		res.emplace_back(p-e*b);
	}
	return res;
}

vector<Point> crosspoint(const Circle &a,const Circle &b){
	if(!intersect(a,b)) return vector<Point>();

	vector<Point> res;
	const Real d = dist(a.c,b.c);

	if(sign(d-(a.r+b.r))==0){
		const Point v = b.c - a.c;
		res.emplace_back(a.c + (v * (a.r /abs(v))));
	}else{
		const Real theta = acos((a.r*a.r+d*d-b.r*b.r)/(2*a.r*d));
		const Real phi = angle(b.c-a.c);
		res.emplace_back(a.c+rotate(Point(a.r,0),phi+theta));
		res.emplace_back(a.c+rotate(Point(a.r,0),phi-theta));
	}
	return res;
}

Real area(const Polygon &P){
	Real res = 0.0;
	for(int i=0;i<P.size();++i){
		res+=cross(P[i],P[(i+1)%P.size()]);
	}
	return abs(res) * 0.5;
}


int main(){
	cout.setf(ios::fixed);
	cout.precision(1);

	int n;
	cin >> n;
	Polygon poly(n);
	rep(i,n){
		Point p;
		cin >> p;
		poly[i]=p;
	}
	
	cout << area(poly) << endl;

	return 0;
}

Measured memory_bytes: 1300