#include <bits/stdc++.h>

using namespace std;
ifstream in ("aria.in");
ofstream out ("aria.out");
const int NMAX=100005;
int n;
float s;
struct punct
{
    int x, y;
};
punct v[NMAX];
int main()
{
    in>>n;
    for(int i=1;i<=n;i++)
    {
        in>>v[i].x>>v[i].y;
    }
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(int i=1;i<=n;i++)
    {
        s+=0.50000*((v[i].x*v[i+1].y)-(v[i+1].x*v[i].y));
    }
    out<<s;
    return 0;
}