#include <bits/stdc++.h>
#define Nmax 100005

using namespace std;

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

struct point
{
    double x, y;
};

int N;
point A[Nmax];
double ans;

int main()
{
    int i;
    fin >> N;
    for(i = 1; i <= N; i++)
        fin >> A[i].x >> A[i].y;
    fin.close();
    A[N + 1] = A[1];
    for(i = 1; i <= N; i++)
        ans += (A[i].x * A[i + 1].y - A[i + 1].x * A[i].y);
    fout << fixed << setprecision(5) << ans / 2 << "\n";
    fout.close();
    return 0;
}