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#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
double rez{};
vector<pair<double, double>> date(n+1);
for (int i = 0; i < n; i++)
{
auto& elem = date[i];
cin >> elem.first >> elem.second;
}
date[n] = date[0];
for (int i = 0; i < n; i++)
{
rez += date[i].first * date[i+1].second
- date[i].second * date[i+1].first;
}
rez = rez / 2;
cout << fixed << setprecision(5) << rez;
}