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#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
#define pb push_back
ifstream f("aria.in");
ofstream g("aria.out");
int n;
double aria;
vector<pair<double, double>> v;
int main(){
f >> n;
for (int i = 0; i < n; i++){
int a, b; f >> a >> b;
v.pb(make_pair(a, b));
}
for (int i = 1; i < n; i++)
aria += (v[i].x * v[i - 1].y - v[i].y * v[i - 1].x);
aria += v[0].x * v[n - 1].y - v[0].y * v[n - 1].x;
aria /= 2.0;
g << fixed << setprecision(5) << abs(aria);
return 0;
}