#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    int n;
    cin >> n;
    vector<int> x(n+1), y(n+1);
    for (int i = 0; i < n; ++i) {
        cin >> x[i] >> y[i];
    }
    x[n] = x[0]; y[n] = y[0];
    ll ans = 0;
    for (int i = 0; i < n; ++i) {
        ans += x[i]*y[i+1]-x[i+1]*y[i];
    }
    cout << abs(ans) << "\n";
    return 0;
}