#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <cassert>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef long long ll;
typedef pair<int,int> PII;
const ll mod=1000000007;
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
// head

typedef double db;
db eps=1e-9;
struct point {
	db x,y;
	point() {}
	point(db x,db y):x(x),y(y) {}
	void read() { scanf("%lf%lf",&x,&y); }
	void print() { printf("%.10f %.10f\n",x,y); }
	db len() { return sqrt(x*x+y*y); }
	db len2() { return x*x+y*y; }
};
point operator + (const point &a,const point &b) { return point(a.x+b.x,a.y+b.y);}
point operator - (const point &a,const point &b) { return point(a.x-b.x,a.y-b.y);}
point operator * (const point &a,const db &b) { return point(a.x*b,a.y*b);}
point operator / (const point &a,const db &b) { return point(a.x/b,a.y/b);}
db dot(const point &a,const point &b) { return a.x*b.x+a.y*b.y; }
db det(const point &a,const point &b) { return a.x*b.y-a.y*b.x; }
struct line {
	point a,b;
	line() {}
	line(point a,point b):a(a),b(b) {}
};
point proj(const line &a,const point &b) {
	return a.a+(a.b-a.a)*dot(a.b-a.a,b-a.a)/(a.b-a.a).len2();
}
point reflect(const line &a,const point &b) {
	return proj(a,b)*2-b;
}
int sign(db x) {
	return fabs(x)<eps?0:(x>0?1:-1);
}
int relation(const line &a,const point &b) {
	int x=sign(det(a.b-a.a,b-a.a));
	if (x==1) return 0; // COUNTER_CLOCKWISE ?????¶???
	else if (x==-1) return 1; // CLOCKWISE ?????¶???
	else {
		x=sign(dot(b-a.a,a.b-a.a));
		if (x==-1) return 2; // ONLINE_BACK
		else {
			x=sign(dot(b-a.b,a.b-a.a));
			if (x==1) return 3; // ONLINE_FRONT
			else return 4; // ON_SEGMENT
		}
	}
}
bool parallel(const line &a,const line &b) { return sign(det(a.a-a.b,b.a-b.b))==0;}
bool ismiddle(const line &a,const point &b) {
	return sign(dot(b-a.a,a.b-a.a))>=0&&sign(dot(b-a.b,a.b-a.a))<=0;
}
int crossSS(const line &a,const line &b) {
	if (sign(min(b.a.x,b.b.x)-max(a.a.x,a.b.x))==1) return 0;
	if (sign(max(b.a.x,b.b.x)-min(a.a.x,a.b.x))==-1) return 0;
	if (sign(min(b.a.y,b.b.y)-max(a.a.y,a.b.y))==1) return 0;
	if (sign(max(b.a.y,b.b.y)-min(a.a.y,a.b.y))==-1) return 0;
	int d1=sign(det(a.b-a.a,b.a-a.a))*sign(det(a.b-a.a,b.b-a.a));
	int d2=sign(det(b.b-b.a,a.a-b.a))*sign(det(b.b-b.a,a.b-b.a));
	if (d1==-1&&d2==-1) return 2; // ?§??????????
	else if (d1<=0&&d2<=0) return 1; // ????§??????????
	else return 0; // ?????????
}
point crosspoint(const line &a,const line &b) {
	double a1=det(b.b-b.a,a.a-b.a),a2=det(b.b-b.a,a.b-b.a);
	return a.a*(a2/(a2-a1))-a.b*(a1/(a2-a1));
}
double distSP(const line &a,const point &b) {
	point h=proj(a,b);
	if (ismiddle(a,h)) return (b-h).len();
	else return min((b-a.a).len(),(b-a.b).len());
}
double distSS(const line &a,const line &b) {
	if (crossSS(a,b)>=1) return 0;
	else return min(min(distSP(b,a.a),distSP(b,a.b)),min(distSP(a,b.a),distSP(a,b.b)));
}
double area(vector<point> &a) { // counter-clockwise
	double s=0;
	rep(i,0,SZ(a)) s+=det(a[i],a[(i==SZ(a)-1)?0:i+1]);
	return 0.5*s;
}

int n;
vector<point> T;
int main() {
	scanf("%d",&n);
	rep(i,0,n) {
		point p1;
		p1.read();
		T.pb(p1);
	}
	printf("%.1f\n",area(T));
}

Measured memory_bytes: 2760