#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) REP(i, 0, n)
#define REP(i, s, e) for(int i = (int)(s); i < (int)(e); i++)
#define pb push_back

#define X real()
#define Y imag()
#define x(p) real(p)
#define y(p) real(p)
#define curr(P, i) P[i]
#define next(P, i) P[(i+1)%P.size()]

const double EPS = 1e-8;
const double INF = 1e12;
const double PI = acos(-1);
typedef complex<double> P;
typedef complex<double> point;
namespace std {
	bool operator < (const P& a, const P& b) {
		return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
	}
}
double cross(const P& a, const P& b) {
	return imag(conj(a)*b);
}
double dot(const P& a, const P& b) {
	return real(conj(a)*b);
}

struct L : public vector<P> {
	L(const P &a, const P &b) {
		push_back(a); push_back(b);
	}
};

typedef vector<P> G;
typedef vector<P> polygon;

struct C {
	P p; double r;
	C(const P &p, double r) : p(p), r(r) { }
};
//??????????????????
int ccw(P a, P b, P c) {
	b -= a; c -= a;
	if (cross(b, c) > 0)   return +1;       // counter clockwise
	if (cross(b, c) < 0)   return -1;       // clockwise
	if (dot(b, c) < 0)     return +2;       // c--a--b on line
	if (norm(b) < norm(c)) return -2;       // a--b--c on line
	return 0;
}

bool intersectLL(const L &l, const L &m) {
	return abs(cross(l[1]-l[0], m[1]-m[0])) > EPS || // non-parallel
			abs(cross(l[1]-l[0], m[0]-l[0])) < EPS;   // same line
}
bool intersectLS(const L &l, const L &s) {
	return cross(l[1]-l[0], s[0]-l[0])*       // s[0] is left of l
			cross(l[1]-l[0], s[1]-l[0]) < EPS; // s[1] is right of l
}
bool intersectLP(const L &l, const P &p) {
	return abs(cross(l[1]-p, l[0]-p)) < EPS;
}
bool intersectSS(const L &s, const L &t) {
  return ccw(s[0],s[1],t[0])*ccw(s[0],s[1],t[1]) <= 0 &&
         ccw(t[0],t[1],s[0])*ccw(t[0],t[1],s[1]) <= 0;
}
bool intersectSP(const L &s, const P &p) {
	return abs(s[0]-p)+abs(s[1]-p)-abs(s[1]-s[0]) < EPS; // triangle inequality
}
P projection(const L &l, const P &p) {
	double t = dot(p-l[0], l[0]-l[1]) / norm(l[0]-l[1]);
	return l[0] + t*(l[0]-l[1]);
}
P reflection(const L &l, const P &p) {
	return p + 2.0 * (projection(l, p) - p);
}
double distanceLP(const L &l, const P &p) {
	return abs(p - projection(l, p));
}
double distanceLL(const L &l, const L &m) {
	return intersectLL(l, m) ? 0 : distanceLP(l, m[0]);
}
double distanceLS(const L &l, const L &s) {
	if (intersectLS(l, s)) return 0;
	return min(distanceLP(l, s[0]), distanceLP(l, s[1]));
}
double distanceSP(const L &s, const P &p) {
	const P r = projection(s, p);
	if (intersectSP(s, r)) return abs(r - p);
	return min(abs(s[0] - p), abs(s[1] - p));
}
double distanceSS(const L &s, const L &t) {
	if (intersectSS(s, t)) return 0;
	return min(min(distanceSP(s, t[0]), distanceSP(s, t[1])),
			min(distanceSP(t, s[0]), distanceSP(t, s[1])));
}
P crosspoint(const L &l, const L &m) {
	double A = cross(l[1] - l[0], m[1] - m[0]);
	double B = cross(l[1] - l[0], l[1] - m[0]);
	if (abs(A) < EPS && abs(B) < EPS) return m[0]; // same line
	if (abs(A) < EPS) assert(false); // !!!PRECONDITION NOT SATISFIED!!!
	return m[0] + B / A * (m[1] - m[0]);
}
enum { OUT, ON, IN };
int contains(const G& pol, const P& p){
	if(pol.size() < 2) return OUT;
	if(intersectSP({pol[0], pol[1]}, p)) return ON;
	bool f = cross(pol[1]-pol[0], p - pol[0]) > 0;
	for(int i = 1; i < pol.size(); i++) {
		auto& now = curr(pol, i);
		auto& nxt = next(pol, i);
		if(intersectSP({nxt, now}, p)) return ON;
		if(f^(cross(nxt-now, p-now)>0)) return OUT;
	}
	return IN;
}
bool contains(const polygon& a, const polygon& b) {
	rep(i, a.size()) if(contains(b, a[i]) == OUT) return false;
	return true;
}
//????????´???????£???????????????????(??????????????£???????????????)
enum{NO_HIT, HIT, A_IN_B, B_IN_A};
int isHit(const polygon& a, const polygon& b){
	if(contains(a, b)) return A_IN_B;
	if(contains(b, a)) return B_IN_A;
	rep(i, a.size()) rep(j, b.size()){
		L la(curr(a, i), next(a, i)), lb(curr(b, j), next(b, j));
		if(intersectSS(la, lb)) return HIT;
	}
	return NO_HIT;
}

//point???theta???????????¢
point spinP(const point& p, double theta, double ext = 1.0) {
	return p*P(cos(theta),sin(theta))*ext;
}

point makeP(double r, double arg) {return P(r*cos(arg), r*sin(arg));}

//?????¨????????????????±???????
vector<P> crossPoint(const C& a, const C& b){
	double d = abs(b.p-a.p);
	double arg = atan2(y(b.p-a.p), x(b.p-a.p));
	double arg2 = acos((d*d+a.r*a.r-b.r*b.r)/(2*d*a.r));
	if(abs(arg2)<EPS) return {a.r*makeP(1.0, arg)+a.p};
	if(abs(a.r-b.r)<d && d < a.r+b.r) return {a.r*makeP(1.0, arg+arg2)+a.p, a.r*makeP(1.0, arg-arg2)+a.p};
	return {};
}

//????????±?????\???
vector<L> comonTang(C c1, C c2) {
	vector<L> ret;
	if(c1.r > c2.r) swap(c1, c2);
	P sub = c1.p;
	c1.p -= sub;
	c2.p -= sub;
	double theta = atan2(c2.p.Y, c2.p.X);

	c2.p = spinP(c2.p, -theta);
	int signX = 0;
	if(c2.p.X > 0) signX = 1;
	if(c2.p.X < 0) signX = -1;

	double x = abs(c2.p.X), r1 = c1.r, r2 = c2.r;
	if(x*x > (r2 - r1) * (r2 - r1)) {
		double theta2 = asin((r2-r1)/x);
		ret.pb({spinP({0, r1}, theta2), spinP({sqrt(x*x-(r2-r1)*(r2-r1)), r1}, theta2)});
		ret.pb({spinP({0, -r1}, -theta2), spinP({sqrt(x*x-(r2-r1)*(r2-r1)), -r1}, -theta2)});
	}
	if(x > r1+r2) {
		double theta3 = PI / 2 - acos((r1+r2)/x);
		double len = sqrt((r1*x)*(r1*x)/(r1+r2)/(r1+r2)-r1*r1) + sqrt((r2*x)*(r2*x)/(r1+r2)/(r1+r2)-r2*r2);
		ret.pb({spinP({0, r1}, -theta3), spinP({len, r1}, -theta3)});
		ret.pb({spinP({0, -r1}, -theta3), spinP({len, -r1}, theta3)});
	}

	for(auto& l : ret) {
		if(signX == -1) l[1] = {-l[1].X, l[1].Y};
		l[0] = spinP(l[0], theta);
		l[1] = spinP(l[1], theta);
		l[0] += sub;
		l[1] += sub;
	}
	return ret;
}

double getArea(const polygon& pol) {
	int n = pol.size();
	double s = 0.0;
	for(int i = 0; i < n; i++) {
		int now = i, nxt = (i + 1) % n;
		s += (pol[now].X - pol[nxt].X) * (pol[now].Y + pol[nxt].Y);
	}
	return abs(s/2.0);
}



int main() {
	int n;
	cin >> n;
	G g;
	rep(i, n) {
		double x, y;
		cin >> x >> y;
		g.pb({x, y});
	}
	printf("%.1lf\n", getArea(g));
}

Measured memory_bytes: 3208