#include<iostream>
#include<fstream>
#include<cstdio>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
const int NMAX=100009;
int n,i;
long double x[NMAX],y[NMAX];
long double s;
int main()
{
    in>>n;
    for(i=1;i<=n;i++)
    {
        in>>x[i]>>y[i];
        if(i>=2)
        {
            s=(double)(s+(double)((double)(x[i-1]*y[i])-(double)(x[i]*y[i-1])));
        }
    }
    s=(double)(s+(double)((double)(x[n]*y[1])-(double)(x[1]*y[n])));
    s=(double)(s/2);
    out<<s;
}