#include <fstream>
#include <cmath>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct punct {
    double x, y;
}v[100005];
double sol;
int n, i, j;

int main() {
    f >> n;
    for (i = 0; i < n; i++)
        f >> v[i].x>>v[i].y;
    for(i = 0; i < n; i++)
        sol += (v[i].x*v[(i+1)%n].y-v[i].y*v[(i+1)%n].x);
    g << fabs(sol)/2;
    return 0;
}