#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

const int NMax = 100005;
struct node{
    long double x,y;
}v[NMax];

int main()
{
    int n;
    fin >> n;
    for(int i = 1; i <= n; i++){
        fin >> v[i].x >> v[i].y;
    }
    v[n + 1] = v[1];
    long double ans = 0;
    for(int i = 1; i <= n; i++){
        ans = ans + ((v[i].x * v[i + 1].y) - (v[i].y * v[i + 1].x));
    }
    ans /= 2;
    fout << fixed << setprecision(6) << ans;
    return 0;
}