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#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int N;
struct Punct
{
double x, y;
};
Punct v[100001];
int main()
{
f >> N;
for(int i = 0; i < N; ++i)
f >> v[i].x >> v[i].y;
v[N] = v[0];
double ans = 0;
for(int i = 0; i < N; ++i)
ans += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);
g << fixed<<setprecision(8)<< ans / 2.0;
return 0;
}