#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
struct Punct{
    long double x;
    long double y;
}v [100003];
int n;
long double ans;
int main (){
    fin >> n;
    for (int i = 1; i <= n; i ++)
        fin >> v [i].x >> v [i].y;
    v [n + 1].x = v [1].x;
    v [n + 1].y = v [1].y;
    for (int i = 1; i <= n; i ++)
        ans += v [i].x * v [i + 1].y - v [i + 1].x * v [i].y;
    if (ans < 0)ans = -ans;
    fout << fixed << setprecision (5) << ans / 2;
    return 0;
}