#include <bits/stdc++.h>

using namespace std;

long long determinant(pair <int, int> A, pair <int, int> B, pair <int, int> C) {
    return 1LL * A.first * B.second + 1LL * B.first * C.second + 1LL * C.first * A.second -
        1LL * A.second * B.first - 1LL * B.second * C.first - 1LL * C.second * A.first;
}

long long getSegmentArea(pair <int, int> A, pair <int, int> B) {
    return determinant({0, 0}, A, B);
}

pair <int, int> v[100005];

int32_t main() {
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> v[i].first >> v[i].second;
    }
    v[n + 1] = v[1];

    long long aria = 0;
    for (int i = 1; i <= n; ++i) {
        aria += getSegmentArea(v[i], v[i + 1]);
    }

    double result = aria / 2.0;

    cout << fixed << setprecision(6) << result;
}