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

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

int main() {
    int n; cin >> n;
    auto p = vec<tuple<int, int>>(uns, n);
    for (auto &[x, y] : p) cin >> x >> y;

    int64_t sum = 0;

    auto r = p[0];
    for (int i = 2; i < n; ++i) {
        auto [x1, y1] = r;
        auto [x2, y2] = p[i - 1];
        auto [x3, y3] = p[i];

        sum += abs((x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3));
    }

    cout << sum << endl;
}