#include <bits/stdc++.h>

using namespace std;

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

const int DIM =1e5 + 7;

struct punct
{
    double x,y;
}v[DIM];

int main()
{
    int n;
    in >> n;

    for(int i = 0;i < n; ++i)
       in >> v[i].x >> v[i].y;

       double rez;
       v[n] = v[0];
    for(int i = 0; i < n; ++i)
        rez += ((v[i].x * v[i+1].y) - (v[i+1].x * v[i].y));
    out << fabs(rez / 2.0);
    return 0;
}