#include <bits/stdc++.h>

using namespace std;

const int nmax=100001;
typedef pair<float, float> punct;

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

int n;
punct a[nmax];

float metodatrapez()
{
    float suma=0;
    for(int i=0; i<n; i++)
    {
        suma+=(a[i].second+a[i+1].second)*(a[i+1].first-a[i].first);
    }
    return abs(0.5*suma);
}

float determinant(int i, int j)
{
    return a[i].first*a[i+1].second-a[i].second*a[i+1].first;
}

float metodatriunghi()
{
    float suma;
    for(int i=0; i<n; i++)
    {
        suma+=0.5*determinant(i, i+1);
    }
    return abs(suma);
}

int main()
{
    fin>>n;
    for(int i=0; i<n; i++)
    {
        fin>>a[i].first>>a[i].second;
    }
    a[n].first=a[0].first;
    a[n].second=a[0].second;
    fout<<metodatrapez();
    //fout<<metodatriunghi();
    return 0;
}