#include<fstream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<iostream>

using namespace std;

ifstream f("arie.in");
ofstream g("arie.out");

int n,x[100005],y[100005];
float s;

void citire()
{
    int i;
    f>>n;
    for(i=1;i<=n;i++){
        f>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
}

void rez()
{
    int i;
    float part;
    for(i=1;i<=n;i++){
        part=(x[i]*y[i+1]-x[i+1]*y[i])/2;
        s+=part;
    }
    g<<s<<"\n";
}

int main()
{
    citire();
    rez();
    return 0;
}