#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define pii pair<double,double>
using namespace std;
vector<pii>v;
int n;
double x, y;
double ans;
double arr(pii a, pii b)
{
    return (a.first * b.second - a.second * b.first);
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for (int i=1;i<=n;++i)
    {
        scanf("%lf %lf",&x,&y);
        v.pb(mp(x,y));
    }
    v.pb(mp(v[0].first,v[0].second));
    for (int i=0;i<v.size();++i)
    {
        ans += arr(v[i], v[i+1]);
    }
    cout<<ans<<'\n';
    fclose(stdin);
    fclose(stdout);
    return 0;
}