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

// 定義・定数
using P = complex<double>;
using Polygon = vector<P>;
const double EPS = 1e-10;

namespace std {
  // 点の比較演算子
  bool operator < (const P &a, const P &b) {
    return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
  }
  
  // 点の等価演算子
  bool operator == (const P &a, const P &b) {
    return fabs(a - b) < EPS;
  }
}

// 直線
struct L : public vector<P> {
  L(P &_a, P &_b) {
    push_back(_a);
    push_back(_b);
  }
};

// 線分
struct S : L {
  S(P &_a, P &_b) : L(_a, _b) {}
};

// 円
struct C {
  P p;
  double r;
  
  C(P _p, double _r) : p(_p), r(_r) {}
};

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

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

// 直交判定
bool isOrthogonal(const L &l, const L &m) {
  return abs(dot(l[1] - l[0], m[1] - m[0])) < EPS;
}

// 平行判定
bool isParallel(const L &l, const L &m) {
  return abs(cross(l[1] - l[0], m[1] - m[0])) < EPS;
}

// 点の進行方向
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;
}

// 交差判定
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 S &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 S &s, const S &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 S &s, const P &p) {
  return abs(s[0] - p) + abs(s[1] - p) - abs(s[1] - s[0]) < EPS; // triangle inequality
}

// 距離など
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 S &s) {
  if (intersectLS(l, s)) return 0;
  return min(distanceLP(l, s[0]), distanceLP(l, s[1]));
}
double distanceSP(const S &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 S &s, const S &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]);
}

// 多角形の面積
double area(const Polygon &po) {
  double A = 0;
  for (int i = 0; i < po.size(); i++) {
    A += cross(po.at(i), po.at((i + 1) % po.size()));
  }
  return A / 2;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  
  int n;
  cin >> n;
  Polygon po;
  for (int i = 0; i < n; i++) {
    double x, y;
    cin >> x >> y;
    po.emplace_back(complex<double>(x, y));
  }
  cout << fixed << setprecision(1) << area(po) << '\n';
}


Measured memory_bytes: 3308