#include <bits/stdc++.h>

using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
const int nx=100002;
struct punct
{
    double x, y;
};
punct v[nx], p={0,0};
double aria;
double det (punct a, punct b, punct c)
{
    double rez = (a.x*b.y+b.x*c.y+c.x*a.y)-(a.y*b.x+b.y*c.x+c.y*a.x);
    return rez;
}
int n;
int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
        in>>v[i].x>>v[i].y;
    p.x=0;
    p.y=0;
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
        aria+=det(p,v[i],v[i+1]);
    out<<fixed<<setprecision(5)<<aria/2;
    return 0;
}