#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
float ans;
struct points
{
    float x;
    float y;
};
points v[100001];
float aria(points a,points b,points c)
{
    a.x-=c.x;
    a.y-=c.y;
    b.x-=c.x;
    b.y-=c.y;
    return (a.x*b.y-a.y*b.x)/2;
}
int main()
{
    ios_base::sync_with_stdio(false);
    fin.tie(0);
    fout.tie(0);
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(int i=1;i<=n-2;i++)
        ans+=aria(v[1],v[i+1],v[i+2]);
    fout<<setprecision(5)<<ans;
    return 0;
}