#include <bits/stdc++.h>

using namespace std;

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

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

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

    long double result = aria / 2.0;

    cout << fixed << setprecision(10) << result;
}