#include <bits/stdc++.h>

using namespace std;

typedef long double ld;

struct point
{
    ld x;
    ld y;
};

inline ld foo(point a,point b)
{
    return (a.x-b.x)*(a.y+b.y);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n;
    cin>>n;
    vector<point>kol(n);
    for(int i=0;i<n;i++)
    {
        cin>>kol[i].x>>kol[i].y;
    }
    ld ans=0;
    for(int i=0;i<n;i++)
    {
        ans+=foo(kol[i],kol[(i+1)%n]);
    }
    ans*=0.5;
    ans=fabs(ans);
    cout<<fixed<<setprecision(6)<<ans<<"\n";
    return 0;
}