#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <vector>

using flt = double;
const flt eps = 1e-12, inf = 1e18, PI = acos(-1.0);
template<typename T>
inline T sqr(T x) {return x * x;}
inline flt cmp(flt a, flt b, flt e = eps) {
  return fabs(a - b) >= e + fabs(a) * e ? a - b : 0;
}
inline int sgn(flt x, flt e = eps) {return x < -e ? -1 : (x > e);}
inline flt fix(flt x, flt e = eps) {return cmp(x, 0, e);}

struct point {
  flt x, y;
  point(flt x = 0, flt y = 0): x(x), y(y) {}
  bool operator < (const point &rhs) const {
    return cmp(x, rhs.x) < 0 || (cmp(x, rhs.x) == 0 && cmp(y, rhs.y) < 0);
  }
  bool operator == (const point &rhs) const {
    return cmp(x, rhs.x) == 0 && cmp(y, rhs.y) == 0;
  }
  point operator + (const point &rhs) const {
    return point(x + rhs.x, y + rhs.y);
  }
  point operator - (const point &rhs) const {
    return point(x - rhs.x, y - rhs.y);
  }
  point operator * (const flt k) const {
    return point(x * k, y * k);
  }
  point operator / (const flt k) const {
    return point(x / k, y / k);
  }
  point operator ~ () const {// counter clockwise rotate 90 degree
    return point(-y, x);
  }
  flt dot(const point &rhs) const {
    return x * rhs.x + y * rhs.y;
  }
  flt det(const point &rhs) const {
    return x * rhs.y - y * rhs.x;
  }
  flt norm2() const {
    return x * x + y * y;
  }
  flt norm() const {
    return hypot(x, y);
  }
  point rot(flt a) const {// counter clockwise rotate A rad
    return point(x * cos(a) - y * sin(a), x * sin(a) + y * cos(a));
  }
  point rot(flt cosa, flt sina) const {// counter clockwise rotate using cos/sin
    return point(x * cosa - y * sina, x * sina + y * cosa);
  }
  point trunc(flt a = 1.0) const {
    return (*this) * (a / this->norm());
  }
};

using poly_t = std::vector<point>;

flt area(const poly_t &P, point &res) {
  flt sum = 0;
  res = point(0, 0);
  int n = P.size();
  for (int i = 0; i < n; ++i) {
    const point &A = P[i], &B = P[(i + 1) % n];
    sum += B.det(A);
    res = res + (A + B) * B.det(A);
  }
  sum = abs(sum);
  res = res / (3.0 * sum);
  return sum * 0.5;
}

int main() {
  int n;
  scanf("%d", &n);
  std::vector<point> p(n);
  for (int i = 0; i < n; ++i) {
    scanf("%lf%lf", &p[i].x, &p[i].y);
  }
  point mass;
  printf("%.1f\n", area(p, mass));
  return 0;
}