#include <complex>
#include <vector>
#include <map>
#include <iostream>
#include <cstdio>
#include <cassert>

using namespace std;

#define REP(i,n) for(int i = 0; i < (int)(n); ++i)
#define FOR(i,a,b) for(int i = (a); i < (int)(b); ++i)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(v) ((int)v.size())

#define pb push_back
#define mp make_pair
#define mt make_tuple

#define curr(P, i) P[(i) % P.size()]
#define next(P, i) P[(i+1) % P.size()]
#define prev(P, i) P[(i+P.size()-1) % P.size()]

const double EPS = 1e-8;
const double INF = 1e12;

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

// ??????
double dot(const P& a, const P& b) {
    return real(conj(a)*b);
}

// ????¬?????????????2??????
// not verified
double norm(const P& a) {
    return real(conj(a)*a);
}

// ??????
double cross(const P& a, const P& b) {
    return imag(conj(a)*b);
}

// ???p2?????????p0, p1???????????´???????°???±?????????
P projection(const P& p0, const P& p1, const P& p2) {
    // ???????????????????????´???
    P vec1 = p1 - p0;
    P vec2 = p2 - p0;
    return p0 + vec1 * dot(vec1, vec2) / dot(vec1, vec1);
}

// projection?????\??¨???
// not verified
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]);
}

// ???p0, p1???????????´??????????§°?????¨????????????p2??¨????§°??????
P reflection(const P& p0, const P& p1, const P& p2) {
    P proj = projection(p0, p1, p2);
    return proj - (p2 - proj);
}

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;                               // a--c--b on line
}

// 2??´?????????????????????
bool is_parallel(const L& l1, const L& l2) {
    P p1 = l1[1] - l1[0];
    P p2 = l2[1] - l2[0];
    auto res = ccw(p1, p2, P(0,0));
    return (res == 2 || res == -2 || res == 0);
}

// 2??´???????????´?????????
bool is_ortho(const L& l1, const L& l2) {
    P p1 = l1[1] - l1[0];
    P p2 = l2[1] - l2[0];
    return dot(p1, p2) == 0;
}

// l: line(??´???)
// s: segment(??????)
// not verified
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
}

// not verified
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
}

// not verified
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;
}

// not verified
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
}

// not verified
double distanceLP(const L &l, const P &p) {
    return abs(p - projection(l, p));
}

// not verified
double distanceLL(const L &l, const L &m) {
    return intersectLL(l, m) ? 0 : distanceLP(l, m[0]);
}

// not verified
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])));
}

// 2??´????????????
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]);
}

// ????§???¢?????¢???????±???????
// P???????????¨???????????????
double area(const Polygon& poly) {
    double area = 0.0;
    REP(i, SIZE(poly))
        area += cross(curr(poly, i), next(poly, i));
    return abs(area) * 0.5;
}

int main(){
    int N;
    cin >> N;
    Polygon poly;
    REP(n, N) {
        double x, y;
        cin >> x >> y;
        poly.pb( P(x,y) );
    }

    // ??????????°???°?????\???1?????¨???????????¶?????????????????¨???
    printf("%.1lf\n", area(poly));
    return 0;
}

Measured memory_bytes: 1252