#include <bits/stdc++.h>
#define x first
#define y second
#define NMAX 100005

using namespace std;

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

int n;
pair <long double, long double> p[NMAX];
long double ans;

int main()
{
    f >> n;
    for(int i = 1; i <= n; i++)
        f >> p[i].x >> p[i].y;

    n++;
    p[n] = p[1];

    for(int i = 1; i <= n; i++)
        ans += (p[i + 1].x * p[i].y - p[i].x * p[i + 1].y);

    if(ans < 0)
        ans = -ans;
    g << setprecision(5) << fixed << ans / 2;

    return 0;
}