#include <bits/stdc++.h>
using namespace std;
struct punct
{
    double x;
    double y;
}v[100005];
double aria(int a, int b, punct A, punct B)
{
    return A.x * B.y - A.y * B.x;
}
double s;
int n, i;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f >> n;
    for(i = 1; i <= n; i ++)
        f >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    for(i = 1; i <= n; i ++)
        s = s + aria(0, 0, v[i], v[i + 1]);
    if(s < 0)g << -s / 2;
    else g << setprecision(6) << fixed << s / 2;
    return 0;
}