#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");
long double s;
int n;
long double xv[100001],yv[100001];

int main()
{
    f>>n;
    f>>xv[0]>>yv[0];
    xv[n]=xv[0];
    yv[n]=yv[0];
    for(int i=1;i<=n;i++)
    {
        f>>xv[i]>>yv[i];
        s=s+(xv[i-1]*yv[i])-(xv[i]*yv[i-1]);
    }

    long double a=s/(long double)2;
    g<<fixed<<setprecision(6)<<a<<endl;;

    return 0;
}