// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define repR(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i, n) for (int i = (int)(n); i >= 1; i--)
#define loop(i, a, B) for (int i = a; i B; i++)
#define loopR(i, a, B) for (int i = a; i B; i--)
#define all(x) begin(x), end(x)
#define allR(x) rbegin(x), rend(x)
#define rng(x, l, r) begin(x) + (l), begin(x) + (r)
#define pb push_back
#define eb emplace_back
#define fst first
#define snd second
template <class A, class B> constexpr auto mp(A &&a, B &&b) { return make_pair(forward<A>(a), forward<B>(b)); }
template <class... T> constexpr auto mt(T&&... x) { return make_tuple(forward<T>(x)...); }
template <class Int> auto constexpr inf_ = numeric_limits<Int>::max()/2-1;
auto constexpr INF32 = inf_<int32_t>;
auto constexpr INF64 = inf_<int64_t>;
auto constexpr INF   = inf_<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T, class Comp> struct pque : priority_queue<T, vector<T>, Comp> { vector<T> &data() { return this->c; } void clear() { this->c.clear(); } };
template <class T> using pque_max = pque<T, less<T>>;
template <class T> using pque_min = pque<T, greater<T>>;
template <class T, class = typename T::iterator, enable_if_t<!is_same<T, string>::value, int> = 0>
ostream& operator<<(ostream& os, T const& a) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, size_t N, enable_if_t<!is_same<T, char>::value, int> = 0>
ostream& operator<<(ostream& os, const T (&a)[N]) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = decltype(begin(declval<T&>())), class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T, S> const& p) { return os << p.first << " " << p.second; }
template <class T, class S> istream& operator>>(istream& is, pair<T, S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint { template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); } };
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T, d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T, d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T, 0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T, d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T, d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T, class U> constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; }
template <class T, class U> constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
template <class C, class T> int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); }
template <class C, class T> int ubd(C const& v, T const& x) { return upper_bound(begin(v), end(v), x)-begin(v); }
constexpr int64_t mod(int64_t x, int64_t m) { assert(m > 0); return (x %= m) < 0 ? x+m : x; }
constexpr int64_t div_floor(int64_t x, int64_t y) { assert(y != 0); return x/y - ((x^y) < 0 and x%y); }
constexpr int64_t div_ceil(int64_t x, int64_t y) { assert(y != 0); return x/y + ((x^y) > 0 and x%y); }
constexpr int dx[] = { 1, 0, -1, 0, 1, -1, -1, 1 };
constexpr int dy[] = { 0, 1, 0, -1, 1, 1, -1, -1 };
constexpr int popcnt(ll x) { return __builtin_popcountll(x); }
mt19937_64 seed_{random_device{}()};
template <class Int> Int rand(Int a, Int b) { return uniform_int_distribution<Int>(a, b)(seed_); }
i64 irand(i64 a, i64 b) { return rand<i64>(a, b); } // [a, b]
u64 urand(u64 a, u64 b) { return rand<u64>(a, b); } //
template <class It> void shuffle(It l, It r) { shuffle(l, r, seed_); }
vector<int> &operator--(vector<int> &v) { for (int &x : v) --x; return v; }
vector<int> &operator++(vector<int> &v) { for (int &x : v) ++x; return v; }
// <<<
// >>> Point
template <class T> constexpr bool equal(T x, T y) { return x == y; }
template <class T> struct Point {
    T x, y;
    constexpr Point() : x(0), y(0) {}
    constexpr Point(T x, T y) : x(x), y(y) {}
    constexpr Point(complex<T> const& z) : x(z.real()), y(z.imag()) {}
    constexpr pair<T, T> to_pair() const { return {x, y}; }
    constexpr Point inv() const { return {x/norm(), -y/norm()}; }
    constexpr Point conj() const { return {x, -y}; }
    constexpr T norm() const { return x*x + y*y; }
    constexpr Point rot90(int n = 1) const {
        n %= 4; if (n < 0) n += 4;
        if (n == 1) return *this * Point(0, 1);
        if (n == 2) return *this * (-1);
        if (n == 3) return *this * Point(0, -1);
        return *this;
    }
    constexpr Point operator+() const { return *this; }
    constexpr Point operator-() const { return {-x, -y}; }
    constexpr Point operator+(Point const& p) const { return {x+p.x, y+p.y}; }
    constexpr Point operator-(Point const& p) const { return {x-p.x, y-p.y}; }
    constexpr Point operator*(Point const& p) const { return {x*p.x-y*p.y, x*p.y+y*p.x}; }
    constexpr Point operator/(Point const& p) const { return *this * p.conj() / p.norm(); }
    constexpr Point operator*(T const& a) const { return {a*x, a*y}; }
    constexpr Point operator/(T const& a) const { return {x/a, y/a}; }
    constexpr Point& operator+=(Point const& p) { return *this = *this + p; }
    constexpr Point& operator-=(Point const& p) { return *this = *this - p; }
    constexpr Point& operator*=(Point const& p) { return *this = *this * p; }
    constexpr Point& operator/=(Point const& p) { return *this = *this / p; }
    constexpr Point& operator*=(T const& a) { return *this = *this * a; }
    constexpr Point& operator/=(T const& a) { return *this = *this / a; }
    constexpr friend Point operator*(T const& a, Point const& p) { return p*a; }
    constexpr friend Point operator/(T const& a, Point const& p) { return Point(a)/p; }
    constexpr T dot(Point const& q) const { return x*q.x + y*q.y; }
    constexpr T cross(Point const& q) const { return x*q.y - y*q.x; }
    constexpr T dot(Point const& p, Point const& q) const { return (p-*this).dot(q-*this); }
    constexpr T cross(Point const& p, Point const& q) const { return (p-*this).cross(q-*this); }
    // constexpr bool operator==(Point const& q) const { return sgn(x-q.x) == 0 && sgn(y-q.y) == 0; }
    constexpr bool operator==(Point const& q) const { return equal(x, q.x) and equal(y, q.y); }
    constexpr bool operator!=(Point const& q) const { return !operator==(q); }

    constexpr friend Point conj(Point const& p) { return p.conj(); }
    constexpr friend T norm(Point const& p) { return p.x*p.x + p.y*p.y; }
    constexpr friend T dot(Point const& p, Point const& q) { return p.dot(q); }
    constexpr friend T cross(Point const& p, Point const& q) { return p.cross(q); }
#ifdef LOCAL
    friend string to_s(Point const& p) { return to_s(p.to_pair()); }
#endif
    friend istream& operator>>(istream& is, Point& p) { return is >> p.x >> p.y; }
    friend ostream& operator<<(ostream& os, Point const& p) { return os << p.x << " " << p.y; }
};

// <<<
using P = Point<int>;

constexpr int sgn(int x) { return x > 0 ? 1 : x < 0 ? -1 : 0; }

using Real = long double;
constexpr Real eps = 1e-10;
constexpr Real pi = acos(-1.0L);
constexpr Real to_rad(Real deg) { return Real(deg) * pi / 180; }
constexpr Real to_deg(Real rad) { return rad * 180 / pi; }
template <class T> constexpr T sq(T const& x) { return x*x; }
constexpr int sgn(Real x) { return x > eps ? 1 : x < -eps ? -1 : 0; }
template <> constexpr bool equal<Real>(Real x, Real y) { return sgn(x-y) == 0; }
template <class T>
constexpr Real abs(Point<T> const& p) { return sqrt((Real)p.norm()); }
constexpr Point<Real> normalize(Point<Real> const& p) { return p/abs(p); }
//using P = Point<Real>;

int32_t main() {
    int n; cin >> n;
    vector<P> p(n); cin >> p;

    int ans = 0;
    rep (i, n) ans += cross(p[i], p[mod(i+1, n)]);
    cout << ans << '\n';

}