#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int32_t n;
    cin >> n;
    vector<pair<int32_t, int32_t>> xys(n);
    for (auto &&xy : xys) {
        cin >> xy.first >> xy.second;
    }
    int64_t ans = 0;
    for (auto i = 1; i < n - 1; ++i) {
        int64_t x0 = xys[i].first - xys[0].first,
                y0 = xys[i].second - xys[0].second;
        int64_t x1 = xys[i + 1].first - xys[0].first,
                y1 = xys[i + 1].second - xys[0].second;
        ans += abs(x0 * y1 - x1 * y0);
    }
    cout << ans << endl;
    return 0;
}