#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
struct
{
    double x,y;
}v[100005];
double rez;
int main()
{
    fin>>n;
    for (int i=1;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    v[n+1]=v[1];
    for (int i=1;i<=n;i++)
    {
        rez+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    }
    fout<<rez/2;
    return 0;
}