#include <fstream>
#include <vector>
using namespace std;

#define x first
#define y second

ifstream fin("aria.in");
ofstream fout("aria.out");

pair<int, int> p, a;
vector<pair<int, int> > pct;
int n, i;
double aria;

int main() {
    fin >> n;
    fin >> a.x >> a.y;
    pct.push_back(a);
    for (i = 2 ; i <= n ; i++) {
        fin >> p.x >> p.y;
        pct.push_back(p);
    }
    pct.push_back(a);
    for (i = 0 ; i < n ; i++)
        aria += (pct[i].x * pct[i + 1].y - pct[i + 1].x * pct[i].y) * 0.5;
    fout << aria;
}