#include <bits/stdc++.h>

using namespace std;

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

long double xv[100001],yv[100001];
int main()
{
    int n;
    long double s=0;
    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+=xv[i-1]*yv[i]-xv[i]*yv[i-1];
    }
    long double a=s/(long double)2;
    g<<fixed<<setprecision(6)<<a<<"\n";
    return 0;
}