#include <bits/stdc++.h>

using namespace std;

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

const int nMax = 100005;
struct el
{
    double x, y;
};
el a[nMax];
int n;
double ans;

inline void Read()
{
    int i;
    fin >> n;
    for(i = 0; i < n; i++)
        fin >> a[i].x >> a[i].y;
}

inline void Solve()
{
    int i;
    a[n] = a[0];
    for(i = 0; i < n; i++)
        ans += (a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);
    fout << ans / 2.0;
}

int main()
{
    Read();
    Solve();
    return 0;
}