#include <bits/stdc++.h>
using namespace std;

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

int n;

float aria(int x1, int y1, int x2, int y2, int x3, int y3)
{
    return (x1*y2+x2*y3+y1*x3-y2*x3-x2*y1-y3*x1)/2;
}

int main()
{
    fin>>n;
    float s=0;
    pair<int, int> v[n+2];

    for(int i=1; i<=n; i++)
    {
        fin>>v[i].first>>v[i].second;
    }
    for(int i=1; i<=n-2; i++)
    {
        s+=aria(v[i].first, v[i].second, v[i+1].first, v[i+1].second, v[i+2].first, v[i+2].second);
    }
    fout<<s;
    fin.close();
    fout.close();
    return 0;
}