#include <iostream>
#include <fstream>
using namespace std;
ifstream si("aria.in");
ofstream so("aria.out");
double x[100005];
double y[100005];
int main()
{
    int n;
    si>>n;
    double sum=0;
    int i;
    for(i=0;i<n;++i)
    {
        si>>x[i]>>y[i];
    }
    x[n]=x[0];
    y[n]=y[0];
    for(i=0;i<n;++i)
    {
        sum+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    }
    so<<sum<<'\n';
    return 0;
}