#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define ld long double
#define pii pair<ld,ld>
using namespace std;
vector<pii>v;
int n;
ld x, y, ans;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    cin>>n;
    for (int i=1;i<=n;++i)
    {
        cin>>x>>y;
        v.pb(mp(x,y));
    }
    v.pb(mp(v[0].first,v[0].second));
    for (int i=0;i<v.size()-1;++i)
        ans += (v[i].first * v[i+1].second - v[i].second * v[i+1].first);
    //if (ans < 0) ans *= -1.0;
    cout<<setprecision(10)<<fixed<<ans/2.0<<'\n';
    fclose(stdin);
    fclose(stdout);
    return 0;
}