#include <bits/stdc++.h>

using namespace std;

typedef long double ld;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n;
    cin>>n;
    vector<pair<ld,ld>>v(n+1);
    for(int i=0;i<n;i++)
    {
        cin>>v[i].first>>v[i].second;
    }
    v[n]=v[0];
    ld ans=0;
    for(int i=0;i<n;i++)
    {
        ans+=(v[i].first-v[i+1].first)*(v[i].second+v[i+1].second);
    }
    ans=fabs(ans*0.5);
    cout<<fixed<<setprecision(6)<<ans<<"\n";
    return 0;
}