#include <bits/stdc++.h>
#define cin ci
#define cout co
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n, x, y;
vector<pair<int ,int>> v;
long double ans;
int main()
{
    cin >> n;
    v.resize(n+2);
    for(int i=1; i<=n; i++)
        cin >> v[i].first >> v[i].second;
    v[n+1] = v[1];
    for(int i=1; i<=n; i++)
        ans += (long double) (v[i].first * v[i+1].second - v[i].second * v[i+1].first) / 2;
    cout << ans;
    return 0;
}