// >>> TEMPLATES #include 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 constexpr auto mp(A &&a, B &&b) { return make_pair(forward(a), forward(b)); } template constexpr auto mt(T&&... x) { return make_tuple(forward(x)...); } template auto constexpr inf_ = numeric_limits::max()/2-1; auto constexpr INF32 = inf_; auto constexpr INF64 = inf_; auto constexpr INF = inf_; #ifdef LOCAL #include "debug.hpp" #else #define dump(...) (void)(0) #define say(x) (void)(0) #define debug if (0) #endif template struct pque : priority_queue, Comp> { vector &data() { return this->c; } void clear() { this->c.clear(); } }; template using pque_max = pque>; template using pque_min = pque>; template ::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 ::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 = typename enable_if::value>::type> istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; } template ostream& operator<<(ostream& os, pair const& p) { return os << p.first << " " << p.second; } template istream& operator>>(istream& is, pair& p) { return is >> p.first >> p.second; } struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup; template struct FixPoint : private F { constexpr FixPoint(F&& f) : F(forward(f)) {} template constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward(x)...); } }; struct MakeFixPoint { template constexpr auto operator|(F&& f) const { return FixPoint(forward(f)); } }; #define MFP MakeFixPoint()| #define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__) template struct vec_impl { using type = vector::type>; template static type make_v(size_t n, U&&... x) { return type(n, vec_impl::make_v(forward(x)...)); } }; template struct vec_impl { using type = T; static type make_v(T const& x = {}) { return x; } }; template using vec = typename vec_impl::type; template auto make_v(Args&&... args) { return vec_impl::make_v(forward(args)...); } template void quit(T const& x) { cout << x << endl; exit(0); } template constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; } template constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; } template constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits::value_type{}); } template int sz(T const& x) { return x.size(); } template int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); } template 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 Int rand(Int a, Int b) { return uniform_int_distribution(a, b)(seed_); } i64 irand(i64 a, i64 b) { return rand(a, b); } // [a, b] u64 urand(u64 a, u64 b) { return rand(a, b); } // template void shuffle(It l, It r) { shuffle(l, r, seed_); } vector &operator--(vector &v) { for (int &x : v) --x; return v; } vector &operator++(vector &v) { for (int &x : v) ++x; return v; } // <<< // >>> Point template constexpr bool equal(T x, T y) { return x == y; } template struct Point { T x, y; constexpr Point() : x(0), y(0) {} constexpr Point(T x, T y) : x(x), y(y) {} constexpr Point(complex const& z) : x(z.real()), y(z.imag()) {} constexpr pair 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; 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 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 x, Real y) { return sgn(x-y) == 0; } template constexpr Real abs(Point const& p) { return sqrt((Real)p.norm()); } constexpr Point normalize(Point const& p) { return p/abs(p); } //using P = Point; int32_t main() { int n; cin >> n; vector

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