#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;

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