#include <bits/stdc++.h>

using namespace std;

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

struct punct
{
    long double x,y;
};
punct a[100002];
long double arie;
int n;

int main()
{
    in>>n;

    for(int i=1;i<=n;i++)
        in>>a[i].x>>a[i].y;

    a[n+1]=a[1];

    for(int i=1;i<=n;i++)
        arie+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    arie/=2;

    out<<setprecision(5)<<fixed<<arie;

    return 0;
}