#include <bits/stdc++.h>

using namespace std;

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

int n;
struct point
{
    double x, y;
};
double area1(point a, point b, point c) {
a.x -= c.x; b.x -= c.x;
a.y -= c.y; b.y -= c.y;
return fabs((double)(a.x * b.y - b.x * a.y) / 2);
}
point p[100005], a;
int area(int n, point p[]) {
double area_tot = 0;
for (int i = 2; i < n; i++) {
area_tot += area1(p[1], p[i], p[i+1]);
}
return fabs(area_tot);
}

int main()
{
    fin>>n;
    for(int i = 1;i <= n;i++)
    {
        fin>>a.x>>a.y;
        p[i] = a;
    }
    fout<<setprecision(10)<<(double)(area(n, p));
    return 0;
}